Fix crash on signing out with exit warning bubble open.

ash/accelerators/exit_warning_handler.h

 // 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.
 
 #ifndef ASH_ACCELERATORS_EXIT_WARNING_HANDLER_H_
 #define ASH_ACCELERATORS_EXIT_WARNING_HANDLER_H_
 
 #include "ash/ash_export.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/timer/timer.h"
 #include "ui/base/accelerators/accelerator.h"
 
 namespace views {
 class Widget;
 }
 
 namespace ash {
 
 // In order to avoid accidental exits when the user presses the exit
@@ skipping 45 matching lines @@
   // Performs actions when the time limit is exceeded.
   void TimerAction();
 
   void StartTimer();
   void CancelTimer();
 
   void Show();
   void Hide();
 
   State state_;
-  views::Widget* widget_; // owned by |this|.
+  scoped_ptr<views::Widget> widget_;
   base::OneShotTimer<ExitWarningHandler> timer_;
 
   // Flag to suppress starting the timer for testing. For test we call
   // TimerAction() directly to simulate the expiration of the timer.
   bool stub_timer_for_test_;
 
   DISALLOW_COPY_AND_ASSIGN(ExitWarningHandler);
 };
 
 }  // namespace ash
 
 #endif  // ASH_ACCELERATORS_EXIT_WARNING_HANDLER_H_