Makes ash/mus use RootWindowControllerCommon

ash/mus/background_layout.h

-// Copyright 2015 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_MUS_BACKGROUND_LAYOUT_H_
-#define ASH_MUS_BACKGROUND_LAYOUT_H_
-
-#include "ash/mus/layout_manager.h"
-#include "base/macros.h"
-
-namespace ash {
-namespace mus {
-
-// Lays out the desktop background within background containers.
-class BackgroundLayout : public LayoutManager {
- public:
-  explicit BackgroundLayout(::mus::Window* owner);
-  ~BackgroundLayout() override;
-
- private:
-  // Overridden from LayoutManager:
-  void LayoutWindow(::mus::Window* window) override;
-
-  DISALLOW_COPY_AND_ASSIGN(BackgroundLayout);
-};
-
-}  // namespace mus
-}  // namespace ash
-
-#endif  // ASH_MUS_BACKGROUND_LAYOUT_H_