chrome: Cleanup class/struct forward declarations

chrome/browser/task_manager/providers/web_contents/extension_tag.h

 // Copyright 2015 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_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_EXTENSION_TAG_H_
 #define CHROME_BROWSER_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_EXTENSION_TAG_H_
 
 #include "base/macros.h"
 #include "chrome/browser/task_manager/providers/web_contents/extension_task.h"
 #include "chrome/browser/task_manager/providers/web_contents/web_contents_tag.h"
 
-namespace extensions {
-class Extension;
-}  // namespace extensions
-
 namespace task_manager {
 
 // Defines a concrete UserData type for WebContents owned by extensions.
 class ExtensionTag : public WebContentsTag {
  public:
   // task_manager::WebContentsTag:
   ExtensionTask* CreateTask() const override;
 
  private:
   friend class WebContentsTags;
 
   ExtensionTag(content::WebContents* web_contents,
                const extensions::ViewType view_type);
   ~ExtensionTag() override;
 
   // The ViewType of the extension WebContents this tag is attached to.
   const extensions::ViewType view_type_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionTag);
 };
 
 }  // namespace task_manager
 
 #endif  // CHROME_BROWSER_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_EXTENSION_TAG_H_