Clean-up of the system shutdown code paths

ash/system/tray/system_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_TRAY_SYSTEM_TRAY_DELEGATE_H_
 #define ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 187 matching lines @@
 
   // Shows information about enterprise enrolled devices.
   virtual void ShowEnterpriseInfo() = 0;
 
   // Shows information about supervised users.
   virtual void ShowSupervisedUserInfo() = 0;
 
   // Shows login UI to add other users to this session.
   virtual void ShowUserLogin() = 0;
 
-  // Attempts to shut down the system.
-  virtual void ShutDown() = 0;
-
   // Attempts to sign out the user.
   virtual void SignOut() = 0;
 
   // Attempts to lock the screen.
   virtual void RequestLockScreen() = 0;
 
   // Attempts to restart the system for update.
   virtual void RequestRestartForUpdate() = 0;
 
   // Returns a list of available bluetooth devices.
@@ skipping 80 matching lines @@
   virtual void AddCustodianInfoTrayObserver(
       CustodianInfoTrayObserver* observer) = 0;
 
   virtual void RemoveCustodianInfoTrayObserver(
       CustodianInfoTrayObserver* observer) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_