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.

tests/cefclient/browser/client_handler.h

@@ -256,6 +256,7 @@ class ClientHandler : public CefClient,
       CefRefPtr<CefSelectClientCertificateCallback> callback) OVERRIDE;
   void OnRenderProcessTerminated(CefRefPtr<CefBrowser> browser,
                                  TerminationStatus status) OVERRIDE;
+  void OnDocumentAvailableInMainFrame(CefRefPtr<CefBrowser> browser) OVERRIDE;
 
   // CefResourceRequestHandler methods
   cef_return_value_t OnBeforeResourceLoad(
@@ -344,6 +345,8 @@ class ClientHandler : public CefClient,
   void BuildTestMenu(CefRefPtr<CefMenuModel> model);
   bool ExecuteTestMenu(int command_id);
 
+  void SetOfflineState(CefRefPtr<CefBrowser> browser, bool offline);
+
   // THREAD SAFE MEMBERS
   // The following members may be accessed from any thread.
 
@@ -356,6 +359,9 @@ class ClientHandler : public CefClient,
   // True if mouse cursor change is disabled.
   bool mouse_cursor_change_disabled_;
 
+  // True if the browser is currently offline.
+  bool offline_;
+
   // True if Favicon images should be downloaded.
   bool download_favicon_images_;