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

ash/wm/stacking_controller.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_WM_STACKING_CONTROLLER_H_
 #define ASH_WM_STACKING_CONTROLLER_H_
 
 #include "ash/ash_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/aura/client/window_tree_client.h"
 
-namespace aura{
-class RootWindow;
-}
-
 namespace ash {
-
 namespace internal {
 class AlwaysOnTopController;
 }
 
 class ASH_EXPORT StackingController : public aura::client::WindowTreeClient {
  public:
   StackingController();
   virtual ~StackingController();
 
   // Overridden from aura::client::WindowTreeClient:
   virtual aura::Window* GetDefaultParent(aura::Window* context,
                                          aura::Window* window,
                                          const gfx::Rect& bounds) OVERRIDE;
 
  private:
   // Returns corresponding system modal container for a modal window.
   // If screen lock is not active, all system modal windows are placed into the
   // normal modal container.
   // Otherwise those that originate from LockScreen container and above are
   // placed in the screen lock modal container.
-  aura::Window* GetSystemModalContainer(aura::RootWindow* root,
+  aura::Window* GetSystemModalContainer(aura::Window* root,
                                         aura::Window* window) const;
 
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_STACKING_CONTROLLER_H_