[Aura] Support always-on-top top level window.

ui/aura_shell/stacking_controller.h

 // Copyright (c) 2011 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 UI_AURA_SHELL_STACKING_CONTROLLER_H_
 #define UI_AURA_SHELL_STACKING_CONTROLLER_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
 #include "ui/aura/client/stacking_client.h"
 
 namespace aura_shell {
 namespace internal {
 
+class AlwaysOnTopController;
+
 class StackingController : public aura::StackingClient {
  public:
   StackingController();
   virtual ~StackingController();
 
+  // Initializes this controller.
+  void Init();
+
   // Returns true if |window| exists within a container that supports
   // activation.
   static aura::Window* GetActivatableWindow(aura::Window* window);
 
   // Overridden from aura::StackingClient:
   virtual void AddChildToDefaultParent(aura::Window* window) OVERRIDE;
   virtual bool CanActivateWindow(aura::Window* window) const OVERRIDE;
   virtual aura::Window* GetTopmostWindowToActivate(
       aura::Window* ignore) const OVERRIDE;
 
  private:
+  scoped_ptr<internal::AlwaysOnTopController> always_on_top_controller_;
+
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };
 
 }  // namespace internal
 }  // namespace aura_shell
 
 #endif  // UI_AURA_SHELL_STACKING_CONTROLLER_H_