Refactors StackingController::GetDefaultParent to ash/wm/common

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_tree_client.h"
 
 namespace ash {
 class AlwaysOnTopController;
 
 class ASH_EXPORT StackingController : public aura::client::WindowTreeClient {
  public:
   StackingController();
   ~StackingController() override;
 
   // Overridden from aura::client::WindowTreeClient:
   aura::Window* GetDefaultParent(aura::Window* context,
                                  aura::Window* window,
                                  const gfx::Rect& bounds) override;
 
  private:
-  // Returns corresponding system modal container for a modal window.
-  // If screen lock is not active, all system modal windows are placed into the
-  // normal modal container.
-  // Otherwise those that originate from LockScreen container and above are
-  // placed in the screen lock modal container.
-  aura::Window* GetSystemModalContainer(aura::Window* root,
-                                        aura::Window* window) const;
-
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_STACKING_CONTROLLER_H_