Add context to gfx::Screen calls in support of simultaneous desktop+ash

ui/aura/desktop/desktop_screen.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 UI_AURA_DESKTOP_DESKTOP_SCREEN_H_
 #define UI_AURA_DESKTOP_DESKTOP_SCREEN_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace gfx {
-class ScreenImpl;
+class Screen;
 }
 
 namespace aura {
 
-// Creates a ScreenImpl that represents the screen of the environment that hosts
+// Creates a Screen that represents the screen of the environment that hosts
 // a RootWindowHost. Caller owns the result.
-AURA_EXPORT gfx::ScreenImpl* CreateDesktopScreen();
+AURA_EXPORT gfx::Screen* CreateDesktopScreen();
 
 }  // namespace aura
 
 #endif  // UI_AURA_DESKTOP_DESKTOP_SCREEN_H_