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.

libcef_dll/ctocpp/browser_host_ctocpp.h

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=a9c5f9224663a0c2f2a29c35c2dea3f47973b82f$
+// $hash=c9abd1293472afbac964aac4cd7dd4cac9dd8e58$
 //
 
 #ifndef CEF_LIBCEF_DLL_CTOCPP_BROWSER_HOST_CTOCPP_H_
@@ -76,6 +76,12 @@ class CefBrowserHostCToCpp : public CefCToCppRefCounted<CefBrowserHostCToCpp,
                     const CefPoint& inspect_element_at) OVERRIDE;
   void CloseDevTools() OVERRIDE;
   bool HasDevTools() OVERRIDE;
+  bool SendDevToolsMessage(const void* message, size_t message_size) OVERRIDE;
+  int ExecuteDevToolsMethod(int message_id,
+                            const CefString& method,
+                            CefRefPtr<CefDictionaryValue> params) OVERRIDE;
+  CefRefPtr<CefRegistration> AddDevToolsMessageObserver(
+      CefRefPtr<CefDevToolsMessageObserver> observer) OVERRIDE;
   void GetNavigationEntries(CefRefPtr<CefNavigationEntryVisitor> visitor,
                             bool current_only) OVERRIDE;
   void SetMouseCursorChangeDisabled(bool disabled) OVERRIDE;