Add the "virtual" keyword on method overrides that are missing it.

chrome/browser/extensions/extension_message_service.h

 // Copyright (c) 2010 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 CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_SERVICE_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_SERVICE_H_
 #pragma once
 
 #include <map>
 #include <set>
@@ skipping 116 matching lines @@
       const std::string& tab_json,
       const MessagePort& receiver, int receiver_port_id,
       const std::string& source_extension_id,
       const std::string& target_extension_id,
       const std::string& channel_name);
 
   void CloseChannelImpl(MessageChannelMap::iterator channel_iter, int port_id,
                         bool notify_other_port);
 
   // NotificationObserver interface.
-  void Observe(NotificationType type,
-               const NotificationSource& source,
-               const NotificationDetails& details);
+  virtual void Observe(NotificationType type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details);
 
   // An IPC sender that might be in our list of channels has closed.
   void OnSenderClosed(IPC::Message::Sender* sender);
 
   Profile* profile_;
 
   NotificationRegistrar registrar_;
 
   MessageChannelMap channels_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionMessageService);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_SERVICE_H_