Fixes possible crashes for ProfileNotification is not found.

chrome/browser/notifications/message_center_notification_manager.h

Index: chrome/browser/notifications/message_center_notification_manager.h
diff --git a/chrome/browser/notifications/message_center_notification_manager.h b/chrome/browser/notifications/message_center_notification_manager.h
index 8c866386b02e13f95b715f2ad9e5a5f46a35ae07..71d456c018abd1d4c666d5e87e793b335e93fc84 100644
--- a/chrome/browser/notifications/message_center_notification_manager.h
+++ b/chrome/browser/notifications/message_center_notification_manager.h
@@ -143,6 +143,8 @@ class MessageCenterNotificationManager
   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);