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/capi/cef_request_handler_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=a28219cc8c1cb53faacaf236374c3cf2c0c45bef$
+// $hash=0167eb1abe614bd6391d273a8085fa3e53e7c217$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_REQUEST_HANDLER_CAPI_H_
@@ -253,6 +253,14 @@ typedef struct _cef_request_handler_t {
       struct _cef_request_handler_t* self,
       struct _cef_browser_t* browser,
       cef_termination_status_t status);
+
+  ///
+  // Called on the browser process UI thread when the window.document object of
+  // the main frame has been created.
+  ///
+  void(CEF_CALLBACK* on_document_available_in_main_frame)(
+      struct _cef_request_handler_t* self,
+      struct _cef_browser_t* browser);
 } cef_request_handler_t;
 
 #ifdef __cplusplus