fix Screen leak in ash tests

ash/shell.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_SHELL_H_
 #define ASH_SHELL_H_
 
 #include <utility>
 #include <vector>
 
@@ skipping 296 matching lines @@
     return high_contrast_controller_.get();
   }
 
   internal::MagnificationController* magnification_controller() {
     return magnification_controller_.get();
   }
 
   // TODO(oshima): Remove methods that are moved to RootWindowController.
   Launcher* launcher();
 
-  const ScreenAsh* screen() { return screen_; }
+  const ScreenAsh* screen() { return screen_.get(); }
 
   // Force the shelf to query for it's current visibility state.
   void UpdateShelfVisibility();
 
   // Sets/gets the shelf auto-hide behavior.
   void SetShelfAutoHideBehavior(ShelfAutoHideBehavior behavior);
   ShelfAutoHideBehavior GetShelfAutoHideBehavior() const;
 
   void SetShelfAlignment(ShelfAlignment alignment);
   ShelfAlignment GetShelfAlignment();
@@ skipping 76 matching lines @@
 
   // ash::internal::SystemModalContainerEventFilterDelegate overrides:
   virtual bool CanWindowReceiveEvents(aura::Window* window) OVERRIDE;
 
   static Shell* instance_;
 
   // If set before the Shell is initialized, the mouse cursor will be hidden
   // when the screen is initially created.
   static bool initially_hide_cursor_;
 
-  ScreenAsh* screen_;
+  scoped_ptr<ScreenAsh> screen_;
 
   // Active root window. Never becomes NULL during the session.
   aura::RootWindow* active_root_window_;
 
   // The CompoundEventFilter owned by aura::Env object.
   scoped_ptr<aura::shared::CompoundEventFilter> env_filter_;
 
   std::vector<WindowAndBoundsPair> to_restore_;
 
 #if !defined(OS_MACOSX)
@@ skipping 68 matching lines @@
 
   // Used by ash/shell.
   content::BrowserContext* browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(Shell);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_H_