Update NetworkStateNotifier to use message_center.

ash/system/chromeos/network/network_tray_delegate.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 ASH_SYSTEM_CHROMEOS_NETWORK_NETWORK_TRAY_DELEGATE_H
-#define ASH_SYSTEM_CHROMEOS_NETWORK_NETWORK_TRAY_DELEGATE_H
-
-#include "ash/system/chromeos/network/network_observer.h"
-
-namespace ash {
-
-class NetworkTrayDelegate {
- public:
-  virtual ~NetworkTrayDelegate() {}
-
-  // Notifies that the |index|-th link on the notification is clicked.
-  virtual void NotificationLinkClicked(
-      NetworkObserver::MessageType message_type,
-      size_t link_index) = 0;
-};
-
-}  // namespace ash
-
-#endif  // ASH_SYSTEM_CHROMEOS_NETWORK_NETWORK_TRAY_DELEGATE_H