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/cpptoc/dev_tools_message_observer_cpptoc.h

@@ -0,0 +1,40 @@
+// Copyright (c) 2020 The Chromium Embedded Framework Authors. All rights
+// reserved. Use of this source code is governed by a BSD-style license that
+// can be found in the LICENSE file.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool. If making changes by
+// hand only do so within the body of existing method and function
+// implementations. See the translator.README.txt file in the tools directory
+// for more information.
+//
+// $hash=d48f93f3817dab7daf0e30a6fa94b1a173356383$
+//
+
+#ifndef CEF_LIBCEF_DLL_CPPTOC_DEV_TOOLS_MESSAGE_OBSERVER_CPPTOC_H_
+#define CEF_LIBCEF_DLL_CPPTOC_DEV_TOOLS_MESSAGE_OBSERVER_CPPTOC_H_
+#pragma once
+
+#if !defined(WRAPPING_CEF_SHARED)
+#error This file can be included wrapper-side only
+#endif
+
+#include "include/capi/cef_browser_capi.h"
+#include "include/capi/cef_devtools_message_observer_capi.h"
+#include "include/cef_browser.h"
+#include "include/cef_devtools_message_observer.h"
+#include "libcef_dll/cpptoc/cpptoc_ref_counted.h"
+
+// Wrap a C++ class with a C structure.
+// This class may be instantiated and accessed wrapper-side only.
+class CefDevToolsMessageObserverCppToC
+    : public CefCppToCRefCounted<CefDevToolsMessageObserverCppToC,
+                                 CefDevToolsMessageObserver,
+                                 cef_dev_tools_message_observer_t> {
+ public:
+  CefDevToolsMessageObserverCppToC();
+  virtual ~CefDevToolsMessageObserverCppToC();
+};
+
+#endif  // CEF_LIBCEF_DLL_CPPTOC_DEV_TOOLS_MESSAGE_OBSERVER_CPPTOC_H_