make StackingClient dispatch to either ash or desktop

ash/shell/shell_delegate_impl.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 "ash/shell/shell_delegate_impl.h"
 
 #include "ash/caps_lock_delegate_stub.h"
 #include "ash/shell/example_factory.h"
 #include "ash/shell/launcher_delegate_impl.h"
 #include "ash/shell/context_menu.h"
@@ skipping 155 matching lines @@
 }
 
 double ShellDelegateImpl::GetSavedScreenMagnifierScale() {
   return std::numeric_limits<double>::min();
 }
 
 ui::MenuModel* ShellDelegateImpl::CreateContextMenu(aura::RootWindow* root) {
   return new ContextMenu(root);
 }
 
+aura::client::StackingClient* ShellDelegateImpl::CreateStackingClient() {
+  return NULL;
+}
+
 }  // namespace shell
 }  // namespace ash