Implement the Notification `timestamp` property

third_party/WebKit/public/platform/modules/notifications/WebNotificationData.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WebNotificationData_h
 #define WebNotificationData_h
 
 #include "public/platform/WebString.h"
 #include "public/platform/WebURL.h"
 #include "public/platform/WebVector.h"
 #include "public/platform/modules/notifications/WebNotificationAction.h"
 
 namespace blink {
 
 // Structure representing the data associated with a Web Notification.
 struct WebNotificationData {
     enum Direction {
         DirectionLeftToRight,
         DirectionRightToLeft,
         DirectionAuto
     };
 
     WebString title;
     Direction direction = DirectionLeftToRight;
     WebString lang;
     WebString body;
     WebString tag;
     WebURL icon;
     WebVector<int> vibrate;
+    double timestamp = 0;
     bool silent = false;
     bool requireInteraction = false;
     WebVector<char> data;
     WebVector<WebNotificationAction> actions;
 };
 
 } // namespace blink
 
 #endif // WebNotificationData_h