Refactors DimWindow and moves to ash/common

ash/wm/dim_window.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.
-
-#include "ash/ash_export.h"
-#include "base/macros.h"
-#include "ui/aura/window.h"
-#include "ui/aura/window_observer.h"
-
-namespace ash {
-
-// A window used to dim the child windows of the given container.
-class ASH_EXPORT DimWindow : public aura::Window, public aura::WindowObserver {
- public:
-  // Return a dim window for the container if any, or nullptr.
-  static DimWindow* Get(aura::Window* container);
-
-  explicit DimWindow(aura::Window* parent);
-  ~DimWindow() override;
-
-  void SetDimOpacity(float target_opacity);
-
-  // aura::WindowObserver:
-  void OnWindowBoundsChanged(aura::Window* window,
-                             const gfx::Rect& old_bounds,
-                             const gfx::Rect& new_bounds) override;
-  void OnWindowDestroying(aura::Window* window) override;
-
- private:
-  aura::Window* parent_;
-
-  DISALLOW_COPY_AND_ASSIGN(DimWindow);
-};
-
-}  // namespace ash