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

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/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/aura/client/stacking_client.h"
 
 namespace aura{
 class RootWindow;
 }
 
 namespace ash {
+
 namespace internal {
+class AlwaysOnTopController;
+}
 
-class AlwaysOnTopController;
-
-class StackingController : public aura::client::StackingClient {
+class ASH_EXPORT StackingController : public aura::client::StackingClient {
  public:
   StackingController();
   virtual ~StackingController();
 
   // Overridden from aura::client::StackingClient:
-  virtual aura::Window* GetDefaultParent(aura::Window* window,
+  virtual 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::RootWindow* root,
                                         aura::Window* window) const;
 
   // Returns the AlwaysOnTopController of the |root_window|.
   internal::AlwaysOnTopController* GetAlwaysOnTopController(
       aura::RootWindow* root_window);
 
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };
 
-}  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_WM_STACKING_CONTROLLER_H_