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_parser_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=3bc4225f43428d8a3a24dcac1830dafac18b0caf$
+// $hash=14cf03e02d8ca3416e65f756470afd8185c7bc78$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_PARSER_CAPI_H_
@@ -140,6 +140,16 @@ CEF_EXPORT struct _cef_value_t* cef_parse_json(
     const cef_string_t* json_string,
     cef_json_parser_options_t options);
 
+///
+// Parses the specified UTF8-encoded |json| buffer of size |json_size| and
+// returns a dictionary or list representation. If JSON parsing fails this
+// function returns NULL.
+///
+CEF_EXPORT struct _cef_value_t* cef_parse_json_buffer(
+    const void* json,
+    size_t json_size,
+    cef_json_parser_options_t options);
+
 ///
 // Parses the specified |json_string| and returns a dictionary or list
 // representation. If JSON parsing fails this function returns NULL and