chrome: Cleanup class/struct forward declarations

chrome/browser/ui/webui/theme_handler.h

 // Copyright (c) 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_THEME_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_THEME_HANDLER_H_
 
 #include "base/macros.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "content/public/browser/web_ui_message_handler.h"
 
 class Profile;
 
-namespace content {
-class WebUI;
-}
-
 // A class to keep the ThemeSource up to date when theme changes.
 class ThemeHandler : public content::WebUIMessageHandler,
                      public content::NotificationObserver {
  public:
   explicit ThemeHandler();
   ~ThemeHandler() override;
 
  private:
   // content::WebUIMessageHandler implementation.
   void RegisterMessages() override;
 
   // Re/set the CSS caches.
   void InitializeCSSCaches();
 
   // content::NotificationObserver implementation.
   void Observe(int type,
                const content::NotificationSource& source,
                const content::NotificationDetails& details) override;
 
   Profile* GetProfile() const;
 
   content::NotificationRegistrar registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(ThemeHandler);
 };
 
 #endif  // CHROME_BROWSER_UI_WEBUI_THEME_HANDLER_H_