Revert of Deprecate native and extension notification delegates.

chrome/browser/notifications/notification_delegate.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 CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DELEGATE_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DELEGATE_H_
 
 #include <string>
 
+#include "base/memory/ref_counted.h"
 #include "ui/message_center/notification_delegate.h"
 
 // Delegate for a notification. This class has two roles: to implement callback
 // methods for notification, and to provide an identity of the associated
 // notification.
 class NotificationDelegate : public message_center::NotificationDelegate {
  public:
   // Returns unique id of the notification.
   virtual std::string id() const = 0;
 
  protected:
   ~NotificationDelegate() override {}
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_DELEGATE_H_