Implement the Notification `timestamp` property

chrome/browser/notifications/platform_notification_service_impl.cc

 // 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.
 
 #include "chrome/browser/notifications/platform_notification_service_impl.h"
 
 #include "base/command_line.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics_action.h"
 #include "base/prefs/pref_service.h"
@@ skipping 344 matching lines @@
   // different pixel density. Be smarter about this when the API gets updated
   // with a way for developers to specify images of different resolutions.
   Notification notification(
       origin, notification_data.title, notification_data.body,
       gfx::Image::CreateFrom1xBitmap(icon), base::UTF8ToUTF16(origin.host()),
       notification_data.tag, delegate);
 
   notification.set_context_message(
       DisplayNameForContextMessage(profile, origin));
   notification.set_vibration_pattern(notification_data.vibration_pattern);
+  notification.set_timestamp(notification_data.timestamp);
   notification.set_silent(notification_data.silent);
 
   std::vector<message_center::ButtonInfo> buttons;
 
   // Developer supplied buttons.
   for (const auto& action : notification_data.actions)
     buttons.push_back(message_center::ButtonInfo(action.title));
 
 // Android always includes the settings button in all notifications, whereas for
 // desktop only web (not extensions) notifications do.
@@ skipping 61 matching lines @@
         extensions::ExtensionRegistry::Get(profile)->GetExtensionById(
             origin.host(), extensions::ExtensionRegistry::EVERYTHING);
     DCHECK(extension);
 
     return base::UTF8ToUTF16(extension->name());
   }
 #endif
 
   return base::string16();
 }