ash: Cleanup class/struct forward declarations

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/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/aura/client/window_parenting_client.h"
 
 namespace ash {
-class AlwaysOnTopController;
 
 class ASH_EXPORT StackingController
     : public aura::client::WindowParentingClient {
  public:
   StackingController();
   ~StackingController() override;
 
   // Overridden from aura::client::WindowParentingClient:
   aura::Window* GetDefaultParent(aura::Window* context,
                                  aura::Window* window,
                                  const gfx::Rect& bounds) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_STACKING_CONTROLLER_H_