mash: Migrate ScopedTargetRootWindow to //ash/common

ash/scoped_target_root_window.h

-// Copyright 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_SCOPED_TARGET_ROOT_WINDOW_H_
-#define ASH_SCOPED_TARGET_ROOT_WINDOW_H_
-
-#include "ash/ash_export.h"
-#include "base/macros.h"
-
-namespace aura {
-class Window;
-}
-
-namespace ash {
-
-// Constructing a ScopedTargetRootWindow allows temporarily
-// switching a target root window so that a new window gets created
-// in the same window where a user interaction happened.
-// An example usage is to specify the target root window when creating
-// a new window using launcher's icon.
-class ASH_EXPORT ScopedTargetRootWindow {
- public:
-  explicit ScopedTargetRootWindow(aura::Window* root_window);
-  ~ScopedTargetRootWindow();
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(ScopedTargetRootWindow);
-};
-
-}  // namespace ash
-
-#endif  // ASH_SCOPED_TARGET_ROOT_WINDOW_H_