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_api_hash.h

@@ -42,13 +42,13 @@
 // way that may cause binary incompatibility with other builds. The universal
 // hash value will change if any platform is affected whereas the platform hash
 // values will change only if that particular platform is affected.
-#define CEF_API_HASH_UNIVERSAL "a48c939496261acf27f45293484f2c9e4065ca6f"
+#define CEF_API_HASH_UNIVERSAL "eb44c98407c9286a64b8d872f49c772b3fa3eb63"
 #if defined(OS_WIN)
-#define CEF_API_HASH_PLATFORM "7ab8365e6f76e376342247954be2f90f69014776"
+#define CEF_API_HASH_PLATFORM "365b52117c7adf0a8bb6d96638954eb313168ff7"
 #elif defined(OS_MACOSX)
-#define CEF_API_HASH_PLATFORM "01cc20ae8dbd5ce63ff50cac215855c4c58059f4"
+#define CEF_API_HASH_PLATFORM "1d1bca66ca61d742f507c08d4efda9884c94baff"
 #elif defined(OS_LINUX)
-#define CEF_API_HASH_PLATFORM "112ef821732253dfda925d4a1a6d490396bac6d2"
+#define CEF_API_HASH_PLATFORM "a87fd1ea68d4e79c838538fa78d79f8dd05acab2"
 #endif
 
 #ifdef __cplusplus