views: Support top menu switching with mouse and arrow keys (issue #2102)

- cefclient: Add example ViewsMenuBar implementation and show top menu
  by default (specify `--hide-top-menu` to hide it).

include/views/cef_menu_button.h

@@ -78,6 +78,13 @@ class CefMenuButton : public CefLabelButton {
   virtual void ShowMenu(CefRefPtr<CefMenuModel> menu_model,
                         const CefPoint& screen_point,
                         cef_menu_anchor_position_t anchor_position) =0;
+
+  ///
+  // Show the menu for this button. Results in a call to
+  // CefMenuButtonDelegate::OnMenuButtonPressed().
+  ///
+  /*--cef()--*/
+  virtual void TriggerMenu() =0;
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_MENU_BUTTON_H_