Implement the Notification `timestamp` property

content/browser/notifications/notification_database_data.proto

 // Copyright 2015 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package content;
 
@@ skipping 11 matching lines @@
   //
   // Next tag: 3
   message NotificationAction {
     optional string action = 1;
     optional string title = 2;
   }
 
   // Actual data payload of the notification. This message is the protocol
   // buffer meant to serialize the content::PlatformNotificationData structure.
   //
-  // Next tag: 12
+  // Next tag: 13
   message NotificationData {
     enum Direction {
       LEFT_TO_RIGHT = 0;
       RIGHT_TO_LEFT = 1;
       AUTO = 2;
     }
 
     optional string title = 1;
     optional Direction direction = 2;
     optional string lang = 3;
     optional string body = 4;
     optional string tag = 5;
     optional string icon = 6;
     repeated int32 vibration_pattern = 9 [packed=true];
+    optional int64 timestamp = 12;
     optional bool silent = 7;
     optional bool require_interaction = 11;
     optional bytes data = 8;
     repeated NotificationAction actions = 10;
   }
 
   optional NotificationData notification_data = 4;
 }