mac: Minor tweaks to notifications code.

ui/message_center/message_center_observer.h

 // Copyright (c) 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_MESSAGE_CENTER_OBSERVER_H_
 #define UI_MESSAGE_CENTER_MESSAGE_CENTER_OBSERVER_H_
 
+#include <string>
+
 #include "ui/message_center/message_center_export.h"
 
 namespace message_center {
 
 // An observer class for the change of notifications in the MessageCenter.
 class MESSAGE_CENTER_EXPORT MessageCenterObserver {
  public:
   virtual ~MessageCenterObserver() {}
 
   // Called when the notification associated with |notification_id| is added
@@ skipping 19 matching lines @@
                                            int button_index) {}
 
   // Called when the notification associated with |notification_id| is actually
   // displayed.
   virtual void OnNotificationDisplayed(const std::string& notification_id) {}
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_MESSAGE_CENTER_OBSERVER_H_