chrome: Cleanup class/struct forward declarations

chrome/browser/ui/webui/md_feedback/md_feedback_ui.h

 // Copyright 2016 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_MD_FEEDBACK_MD_FEEDBACK_UI_H_
 #define CHROME_BROWSER_UI_WEBUI_MD_FEEDBACK_MD_FEEDBACK_UI_H_
 
 #include "base/macros.h"
 #include "chrome/browser/profiles/profile.h"
 #include "content/public/browser/web_ui_controller.h"
 
 namespace content {
 class BrowserContext;
 }  // namespace content
 
-class MdFeedbackWebUIMessageHandler;
-
 // The WebUI for chrome://feedback.
 class MdFeedbackUI : public content::WebUIController {
  public:
   explicit MdFeedbackUI(content::WebUI* web_ui);
   ~MdFeedbackUI() override;
 
   Profile* profile() { return profile_; }
 
  private:
   Profile* profile_;
 
   DISALLOW_COPY_AND_ASSIGN(MdFeedbackUI);
 };
 
 // Create and show a web dialog with the MD feedback UI.
 // |browser_context| Is used to constructed the HTML dialog's WebContents.
 void ShowFeedbackWebDialog(
     content::BrowserContext* browser_context);
 
 #endif  // CHROME_BROWSER_UI_WEBUI_MD_FEEDBACK_MD_FEEDBACK_UI_H_