Provide default implementations for NotificationDelegate methods.

chrome/browser/chromeos/first_run/drive_first_run_controller.cc

 // 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.
 
 #include "chrome/browser/chromeos/first_run/drive_first_run_controller.h"
 
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "base/callback.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 67 matching lines @@
 // NotificationDelegate for the notification that is displayed when Drive
 // offline mode is enabled automatically. Clicking on the notification button
 // will open the Drive settings page.
 class DriveOfflineNotificationDelegate
     : public message_center::NotificationDelegate {
  public:
   explicit DriveOfflineNotificationDelegate(Profile* profile)
       : profile_(profile) {}
 
   // message_center::NotificationDelegate overrides:
-  virtual void Display() override {}
-  virtual void Error() override {}
-  virtual void Close(bool by_user) override {}
-  virtual void Click() override {}
   virtual void ButtonClick(int button_index) override;
 
  protected:
   virtual ~DriveOfflineNotificationDelegate() {}
 
  private:
   Profile* profile_;
 
   DISALLOW_COPY_AND_ASSIGN(DriveOfflineNotificationDelegate);
 };
@@ skipping 368 matching lines @@
           base::UTF8ToUTF16(extension->name()),
           message_center::NotifierId(message_center::NotifierId::APPLICATION,
                                      kDriveHostedAppId),
           data,
           new DriveOfflineNotificationDelegate(profile_)));
   notification->set_priority(message_center::LOW_PRIORITY);
   message_center::MessageCenter::Get()->AddNotification(notification.Pass());
 }
 
 }  // namespace chromeos