Add API to SendActionToFirstResponder.

It's important to bind application menu items to curtain actions of
first responder, like 'quit', 'minimize', 'copy' etc. This API gives
developers ability to do most of them in javascript.

browser/api/atom_api_menu_mac.h

@@ -31,6 +31,9 @@ class MenuMac : public Menu {
   // submenus and set their titles.
   static void FixMenuTitles(NSMenu* menu);
 
+  // Fake sending an action from the application menu.
+  static void SendActionToFirstResponder(const std::string& action);
+
   DISALLOW_COPY_AND_ASSIGN(MenuMac);
 };