mash: Partially migrate ash/desktop_background to ash common types.

ash/desktop_background/desktop_background_view.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_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_VIEW_H_
 #define ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_VIEW_H_
 
 #include <memory>
 
-#include "base/macros.h"
-#include "ui/events/event.h"
-#include "ui/gfx/image/image_skia.h"
 #include "ui/views/context_menu_controller.h"
 #include "ui/views/view.h"
 
 namespace ash {
 
 class PreEventDispatchHandler;
+class WmWindow;
 
 class DesktopBackgroundView : public views::View,
                               public views::ContextMenuController {
  public:
   DesktopBackgroundView();
   ~DesktopBackgroundView() override;
 
  private:
   friend class DesktopBackgroundControllerTest;
 
   // Overridden from views::View:
   void OnPaint(gfx::Canvas* canvas) override;
   bool OnMousePressed(const ui::MouseEvent& event) override;
 
   // Overridden from views::ContextMenuController:
   void ShowContextMenuForView(views::View* source,
                               const gfx::Point& point,
                               ui::MenuSourceType source_type) override;
   std::unique_ptr<PreEventDispatchHandler> pre_dispatch_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopBackgroundView);
 };
 
+views::Widget* CreateDesktopBackground(WmWindow* root_window, int container_id);
+
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_VIEW_H_