Implement the Notification `timestamp` property

third_party/WebKit/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 50 matching lines @@
     [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 USVString icon;
 
     [RuntimeEnabled=NotificationExperimental] readonly attribute sequence<unsigned long>? vibrate;
+    readonly attribute DOMTimeStamp timestamp;
     readonly attribute boolean silent;
     readonly attribute boolean requireInteraction;
     [CallWith=ScriptState, SameObject] readonly attribute any data;
 
     // TODO(johnme): Ship once Blink supports FrozenArray (https://crbug.com/515920)
     // and we've implemented the additional Object.freeze described in
     // https://notifications.spec.whatwg.org/#dom-notification-actions
     [RuntimeEnabled=NotificationExperimental] readonly attribute sequence<NotificationAction> actions;
 
     [MeasureAs=NotificationClosed] void close();
 };