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.

libcef/browser/views/window_impl.h

@@ -122,6 +122,8 @@ class CefWindowImpl
                 cef_menu_anchor_position_t anchor_position);
   void MenuClosed();
 
+  views::Widget* widget() const { return widget_; }
+
  private:
   // Create a new implementation object.
   // Always call Initialize() after creation.