Add Notification.maxActions so websites know how many buttons we support

Source/modules/notifications/Notification.idl

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 26 matching lines @@
     Exposed=(Window,Worker),
     MeasureAs=NotificationCreated,
     RaisesException=Constructor,
     RuntimeEnabled=Notifications,
 ] interface Notification : EventTarget {
     [CallWith=ExecutionContext, MeasureAs=NotificationPermission] static readonly attribute DOMString permission;
     [CallWith=ExecutionContext, MeasureAs=NotificationPermissionRequested] static void requestPermission(optional NotificationPermissionCallback callback);
 
     // FIXME: Implement the Notification.get() method.
 
+    [RuntimeEnabled=NotificationExperimental] static readonly attribute unsigned long maxActions;
+
     attribute EventHandler onclick;
     [MeasureAs=NotificationShowEvent] attribute EventHandler onshow;
     attribute EventHandler onerror;
     [MeasureAs=NotificationCloseEvent] attribute EventHandler onclose;
 
     readonly attribute DOMString title;
     readonly attribute DOMString dir;
     readonly attribute DOMString lang;
     readonly attribute DOMString body;
     readonly attribute DOMString tag;
     readonly attribute DOMString icon;
 
     [RuntimeEnabled=NotificationExperimental] readonly attribute sequence<unsigned long>? vibrate;
     readonly attribute boolean silent;
     [CallWith=ScriptState] readonly attribute any data;
 
     [MeasureAs=NotificationClosed] void close();
 };