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/browser/browser_window.cc

@@ -73,6 +73,11 @@ void BrowserWindow::OnSetFullscreen(bool fullscreen) {
   delegate_->OnSetFullscreen(fullscreen);
 }
 
+void BrowserWindow::OnAutoResize(const CefSize& new_size) {
+  REQUIRE_MAIN_THREAD();
+  delegate_->OnAutoResize(new_size);
+}
+
 void BrowserWindow::OnSetLoadingState(bool isLoading,
                                       bool canGoBack,
                                       bool canGoForward) {