make StackingClient dispatch to either ash or desktop

ui/aura/desktop/desktop_stacking_client.cc

 // 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.
 
 #include "ui/aura/desktop/desktop_stacking_client.h"
 
 #include "ui/aura/client/default_capture_client.h"
 #include "ui/aura/focus_manager.h"
 #include "ui/aura/root_window.h"
 #include "ui/aura/window.h"
 
 namespace aura {
 
 DesktopStackingClient::DesktopStackingClient() {
-  aura::client::SetStackingClient(this);
 }
 
 DesktopStackingClient::~DesktopStackingClient() {
   aura::client::SetStackingClient(NULL);
 }
 
-Window* DesktopStackingClient::GetDefaultParent(Window* window,
+Window* DesktopStackingClient::GetDefaultParent(Window* context,
+                                                Window* window,
                                                 const gfx::Rect& bounds) {
   if (!null_parent_.get()) {
     null_parent_.reset(new aura::RootWindow(
         aura::RootWindow::CreateParams(gfx::Rect(100, 100))));
     null_parent_->Init();
     null_parent_->set_focus_manager(new FocusManager);
 
     capture_client_.reset(
         new aura::client::DefaultCaptureClient(null_parent_.get()));
   }
   return null_parent_.get();
 }
 
 }  // namespace aura