ui: Cleanup class/struct forward declarations

ui/views/controls/menu/menu_runner_impl_adapter.h

 // Copyright 2014 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_VIEWS_CONTROLS_MENU_MENU_RUNNER_IMPL_ADAPTER_H_
 #define UI_VIEWS_CONTROLS_MENU_MENU_RUNNER_IMPL_ADAPTER_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "ui/views/controls/menu/menu_runner_impl_interface.h"
 #include "ui/views/views_export.h"
 
 namespace views {
+
+class MenuModelAdapter;
+
 namespace internal {
 
 class MenuRunnerImpl;
 
 // Given a MenuModel, adapts MenuRunnerImpl which expects a MenuItemView.
 class VIEWS_EXPORT MenuRunnerImplAdapter
     : public NON_EXPORTED_BASE(MenuRunnerImplInterface) {
  public:
   MenuRunnerImplAdapter(ui::MenuModel* menu_model,
                         const base::Closure& on_menu_closed_callback);
@@ skipping 15 matching lines @@
   std::unique_ptr<MenuModelAdapter> menu_model_adapter_;
   MenuRunnerImpl* impl_;
 
   DISALLOW_COPY_AND_ASSIGN(MenuRunnerImplAdapter);
 };
 
 }  // namespace internal
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_MENU_MENU_RUNNER_IMPL_ADAPTER_H_