Add support for direct DevTools protocol messaging (fixes issue #2961).

This change allows the client to directly send and receive DevTools
protocol messages (send method calls, and receive method results and
events) without requiring a DevTools front-end or remote-debugging
session.

This change includes additional supporting changes:
- Add a new CefRequestHandler::OnDocumentAvailableInMainFrame
  callback (see issue #1454).
- Add a CefParseJSON variant that accepts a UTF8-encoded buffer.
- Add a `--devtools-protocol-log-file=<path>` command-line flag for
  logging protocol messages sent to/from the DevTools front-end
  while it is displayed. This is useful for understanding existing
  DevTools protocol usage.
- Add a new "libcef_static_unittests" executable target to support
  light-weight unit tests of libcef_static internals (e.g. without
  requiring exposure via the CEF API). Files to be unittested are
  placed in the new "libcef_static_unittested" source_set which is
  then included by both the existing libcef_static library and the
  new unittests executable target.
- Linux: Remove use_bundled_fontconfig=false, which is no longer
  required and causes unittest build errors (see issue #2424).

This change also adds a cefclient demo for configuring offline mode
using the DevTools protocol (fixes issue #245). This is controlled
by the "Offline mode" context menu option and the `--offline`
command-line switch which will launch cefclient in offline mode. When
cefclient is offline all network requests will fail with
ERR_INTERNET_DISCONNECTED and navigator.onLine will return false when
called from JavaScript in any frame. This mode is per-browser so
newly created browser windows will have the default mode. Note that
configuring offline mode in this way will not update the Network tab
UI ("Throtting" option) in a displayed DevTools front-end instance.

include/cef_browser.h

@@ -40,10 +40,12 @@
 
 #include <vector>
 #include "include/cef_base.h"
+#include "include/cef_devtools_message_observer.h"
 #include "include/cef_drag_data.h"
 #include "include/cef_frame.h"
 #include "include/cef_image.h"
 #include "include/cef_navigation_entry.h"
+#include "include/cef_registration.h"
 #include "include/cef_request_context.h"
 
 class CefBrowserHost;
@@ -517,6 +519,69 @@ class CefBrowserHost : public virtual CefBaseRefCounted {
   /*--cef()--*/
   virtual bool HasDevTools() = 0;
 
+  ///
+  // Send a method call message over the DevTools protocol. |message| must be a
+  // UTF8-encoded JSON dictionary that contains "id" (int), "method" (string)
+  // and "params" (dictionary, optional) values. See the DevTools protocol
+  // documentation at https://chromedevtools.github.io/devtools-protocol/ for
+  // details of supported methods and the expected "params" dictionary contents.
+  // |message| will be copied if necessary. This method will return true if
+  // called on the UI thread and the message was successfully submitted for
+  // validation, otherwise false. Validation will be applied asynchronously and
+  // any messages that fail due to formatting errors or missing parameters may
+  // be discarded without notification. Prefer ExecuteDevToolsMethod if a more
+  // structured approach to message formatting is desired.
+  //
+  // Every valid method call will result in an asynchronous method result or
+  // error message that references the sent message "id". Event messages are
+  // received while notifications are enabled (for example, between method calls
+  // for "Page.enable" and "Page.disable"). All received messages will be
+  // delivered to the observer(s) registered with AddDevToolsMessageObserver.
+  // See CefDevToolsMessageObserver::OnDevToolsMessage documentation for details
+  // of received message contents.
+  //
+  // Usage of the SendDevToolsMessage, ExecuteDevToolsMethod and
+  // AddDevToolsMessageObserver methods does not require an active DevTools
+  // front-end or remote-debugging session. Other active DevTools sessions will
+  // continue to function independently. However, any modification of global
+  // browser state by one session may not be reflected in the UI of other
+  // sessions.
+  //
+  // Communication with the DevTools front-end (when displayed) can be logged
+  // for development purposes by passing the
+  // `--devtools-protocol-log-file=<path>` command-line flag.
+  ///
+  /*--cef()--*/
+  virtual bool SendDevToolsMessage(const void* message,
+                                   size_t message_size) = 0;
+
+  ///
+  // Execute a method call over the DevTools protocol. This is a more structured
+  // version of SendDevToolsMessage. |message_id| is an incremental number that
+  // uniquely identifies the message (pass 0 to have the next number assigned
+  // automatically based on previous values). |method| is the method name.
+  // |params| are the method parameters, which may be empty. See the DevTools
+  // protocol documentation (linked above) for details of supported methods and
+  // the expected |params| dictionary contents. This method will return the
+  // assigned message ID if called on the UI thread and the message was
+  // successfully submitted for validation, otherwise 0. See the
+  // SendDevToolsMessage documentation for additional usage information.
+  ///
+  /*--cef(optional_param=params)--*/
+  virtual int ExecuteDevToolsMethod(int message_id,
+                                    const CefString& method,
+                                    CefRefPtr<CefDictionaryValue> params) = 0;
+
+  ///
+  // Add an observer for DevTools protocol messages (method results and events).
+  // The observer will remain registered until the returned Registration object
+  // is destroyed. See the SendDevToolsMessage documentation for additional
+  // usage information.
+  ///
+  /*--cef()--*/
+  virtual CefRefPtr<CefRegistration> AddDevToolsMessageObserver(
+      CefRefPtr<CefDevToolsMessageObserver> observer) = 0;
+
   ///
   // Retrieve a snapshot of current navigation entries as values sent to the
   // specified visitor. If |current_only| is true only the current navigation