chrome: Cleanup class/struct forward declarations

chrome/browser/ui/webui/components_ui.h

 // Copyright 2013 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 CHROME_BROWSER_UI_WEBUI_COMPONENTS_UI_H_
 #define CHROME_BROWSER_UI_WEBUI_COMPONENTS_UI_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "components/component_updater/component_updater_service.h"
 #include "components/update_client/crx_update_item.h"
 #include "content/public/browser/web_ui_controller.h"
 #include "ui/base/layout.h"
 
 namespace base {
 class RefCountedMemory;
 }
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 class ComponentsUI : public content::WebUIController,
                      public component_updater::ServiceObserver {
  public:
   explicit ComponentsUI(content::WebUI* web_ui);
   ~ComponentsUI() override;
 
   static void OnDemandUpdate(const std::string& component_id);
 
   static base::ListValue* LoadComponents();
 
   static base::RefCountedMemory* GetFaviconResourceBytes(
       ui::ScaleFactor scale_factor);
 
   // ServiceObserver implementation.
   void OnEvent(Events event, const std::string& id) override;
 
  private:
   static base::string16 ComponentEventToString(Events event);
   static base::string16 ServiceStatusToString(
       update_client::CrxUpdateItem::State state);
   DISALLOW_COPY_AND_ASSIGN(ComponentsUI);
 };
 
 #endif  // CHROME_BROWSER_UI_WEBUI_COMPONENTS_UI_H_