ui: Cleanup class/struct forward declarations

ui/app_list/views/app_list_background.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 UI_APP_LIST_VIEWS_APP_LIST_BACKGROUND_H_
 #define UI_APP_LIST_VIEWS_APP_LIST_BACKGROUND_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/views/background.h"
 
 namespace views {
 class View;
 }
 
 namespace app_list {
 
-class AppListMainView;
-
 // A class to paint bubble background.
 class AppListBackground : public views::Background {
  public:
   explicit AppListBackground(int corner_radius);
   ~AppListBackground() override;
 
  private:
   // views::Background overrides:
   void Paint(gfx::Canvas* canvas, views::View* view) const override;
 
   const int corner_radius_;
 
   DISALLOW_COPY_AND_ASSIGN(AppListBackground);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_VIEWS_APP_LIST_BACKGROUND_H_