Revert of Refactor backdrop

ash/wm/workspace/backdrop_delegate.h

-// Copyright 2014 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_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_DELEGATE_H_
-#define ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_DELEGATE_H_
-
-#include "ash/ash_export.h"
-
-namespace aura {
-class Window;
-}
-
-namespace ash {
-
-// A delegate which can be set to create and control a backdrop which gets
-// placed below the top level window.
-class ASH_EXPORT BackdropDelegate {
- public:
-  virtual ~BackdropDelegate() {}
-
-  // A window got added to the layout.
-  virtual bool HasBackdrop(aura::Window* window) = 0;
-};
-
-}  // namespace ash
-
-#endif  // ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_DELEGATE_H_