reland of https://codereview.chromium.org/11364053/ w/o removing the per-rootwindow stacking client

ash/shell_delegate.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_SHELL_DELEGATE_H_
 #define ASH_SHELL_DELEGATE_H_
 
 #include <vector>
 
 #include "ash/ash_export.h"
 #include "ash/shell.h"
 #include "base/callback.h"
 #include "base/string16.h"
 #include "base/time.h"
 
 namespace app_list {
 class AppListViewDelegate;
 }
 
 namespace aura {
 class RootWindow;
 class Window;
 namespace client {
+class StackingClient;
 class UserActionClient;
 }
 }
 
 namespace ui {
 class MenuModel;
 }
 
 namespace views {
 class Widget;
@@ skipping 140 matching lines @@
 
   // Saves the zoom scale of the full screen magnifier.
   virtual void SaveScreenMagnifierScale(double scale) = 0;
 
   // Gets a saved value of the zoom scale of full screen magnifier. If a value
   // is not saved, return a negative value.
   virtual double GetSavedScreenMagnifierScale() = 0;
 
   // Creates a menu model of the context for the |root_window|.
   virtual ui::MenuModel* CreateContextMenu(aura::RootWindow* root_window) = 0;
+
+  // Creates the stacking client. Shell takes ownership of the object.
+  virtual aura::client::StackingClient* CreateStackingClient() = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_DELEGATE_H_