Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

tests/cefclient/cefclient_gtk.cc

@@ -119,12 +119,13 @@ int RunMain(int argc, char* argv[]) {
   // Register scheme handlers.
   test_runner::RegisterSchemeHandlers();
 
+  RootWindowConfig window_config;
+  window_config.with_controls =
+      !command_line->HasSwitch(switches::kHideControls);
+  window_config.with_osr = settings.windowless_rendering_enabled ? true : false;
+
   // Create the first window.
-  context->GetRootWindowManager()->CreateRootWindow(
-      !command_line->HasSwitch(switches::kHideControls),  // Show controls.
-      settings.windowless_rendering_enabled ? true : false,
-      CefRect(),       // Use default system size.
-      std::string());  // Use default URL.
+  context->GetRootWindowManager()->CreateRootWindow(window_config);
 
   // Run the message loop. This will block until Quit() is called.
   int result = message_loop->Run();