Fixes possible crashes for ProfileNotification is not found.

chrome/browser/notifications/message_center_notification_manager.h

 // Copyright (c) 2012 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_NOTIFICATIONS_MESSAGE_CENTER_NOTIFICATION_MANAGER_H_
 #define CHROME_BROWSER_NOTIFICATIONS_MESSAGE_CENTER_NOTIFICATION_MANAGER_H_
 
 #include <map>
 #include <string>
 
@@ skipping 125 matching lines @@
   NotificationMap profile_notifications_;
 
   scoped_ptr<MessageCenterSettingsController> settings_controller_;
 
   // Helpers that add/remove the notification from local map and MessageCenter.
   // They take ownership of profile_notification object.
   void AddProfileNotification(ProfileNotification* profile_notification);
   void RemoveProfileNotification(ProfileNotification* profile_notification,
                                  bool by_user);
 
+  // Returns the ProfileNotification for the |id|, or NULL if no such
+  // notification is found.
   ProfileNotification* FindProfileNotification(const std::string& id) const;
 
   DISALLOW_COPY_AND_ASSIGN(MessageCenterNotificationManager);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_MESSAGE_CENTER_NOTIFICATION_MANAGER_H_