Replace more ObserverList with base::ObserverList.

ui/message_center/notification_blocker.h

 // Copyright 2013 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 UI_MESSAGE_CENTER_NOTIFICATION_BLOCKER_H_
 #define UI_MESSAGE_CENTER_NOTIFICATION_BLOCKER_H_
 
 #include "base/observer_list.h"
 #include "ui/message_center/message_center_export.h"
 #include "ui/message_center/notification.h"
@@ skipping 29 matching lines @@
   // When a blocker starts returning false for a notification which is already
   // shown as a popup, the notification should be closed as a popup immediately.
   virtual bool ShouldShowNotificationAsPopup(
       const NotifierId& notifier_id) const = 0;
 
  protected:
   MessageCenter* message_center() { return message_center_; }
   void NotifyBlockingStateChanged();
 
  private:
-  ObserverList<Observer> observers_;
+  base::ObserverList<Observer> observers_;
   MessageCenter* message_center_;  // weak
 };
 
 typedef std::vector<NotificationBlocker*> NotificationBlockers;
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_NOTIFICATION_BLOCKER_H_