Make GetRootWindow() return a Window instead of a RootWindow.

ash/display/root_window_transformers.h

 // Copyright (c) 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_DISPLAY_ROOT_WINDOW_TRANSFORMERS_H_
 #define ASH_DISPLAY_ROOT_WINDOW_TRANSFORMERS_H_
 
 #include "ash/ash_export.h"
 
 namespace aura {
-class RootWindow;
 class RootWindowTransformer;
+class Window;
 }
 
 namespace gfx {
 class Display;
 class Transform;
 }
 
 namespace ash {
 namespace internal {
 class DisplayInfo;
 
 ASH_EXPORT aura::RootWindowTransformer* CreateRootWindowTransformerForDisplay(
-    aura::RootWindow* root,
+    aura::Window* root,
     const gfx::Display& display);
 
 // Creates a RootWindowTransformers for mirror root window.
 // |source_display_info| specifies the display being mirrored,
 // and |mirror_display_info| specifies the display used to
 // mirror the content.
 ASH_EXPORT aura::RootWindowTransformer*
 CreateRootWindowTransformerForMirroredDisplay(
     const DisplayInfo& source_display_info,
     const DisplayInfo& mirror_display_info);
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_ROOT_WINDOW_TRANSFORMERS_H_