Implement the Notification `timestamp` property

third_party/WebKit/Source/modules/notifications/Notification.cpp

 /*
  * Copyright (C) 2013 Google 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 235 matching lines @@
 {
     NavigatorVibration::VibrationPattern pattern;
     pattern.appendRange(m_data.vibrate.begin(), m_data.vibrate.end());
 
     if (!pattern.size())
         isNull = true;
 
     return pattern;
 }
 
+DOMTimeStamp Notification::timestamp() const
+{
+    return m_data.timestamp;
+}
+
 bool Notification::silent() const
 {
     return m_data.silent;
 }
 
 bool Notification::requireInteraction() const
 {
     return m_data.requireInteraction;
 }
 
@@ skipping 101 matching lines @@
 }
 
 DEFINE_TRACE(Notification)
 {
     visitor->trace(m_asyncRunner);
     RefCountedGarbageCollectedEventTargetWithInlineData<Notification>::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink