content: Cleanup class/struct forward declarations

content/browser/web_contents/web_contents_view_child_frame.h

 // Copyright (c) 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 CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_VIEW_CHILD_FRAME_H_
 #define CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_VIEW_CHILD_FRAME_H_
 
 #include "base/macros.h"
 #include "content/browser/renderer_host/render_view_host_delegate_view.h"
 #include "content/browser/web_contents/web_contents_view.h"
 
 namespace content {
 
 class WebContents;
 class WebContentsImpl;
 class WebContentsViewDelegate;
-class BrowserPluginGuest;
 
 class WebContentsViewChildFrame : public WebContentsView,
                                   public RenderViewHostDelegateView {
  public:
   WebContentsViewChildFrame(WebContentsImpl* web_contents,
                             WebContentsViewDelegate* delegate,
                             RenderViewHostDelegateView** delegate_view);
   ~WebContentsViewChildFrame() override;
 
   // WebContentsView implementation --------------------------------------------
@@ skipping 48 matching lines @@
 
   // The delegate ownership is passed to WebContentsView.
   std::unique_ptr<WebContentsViewDelegate> delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(WebContentsViewChildFrame);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_VIEW_CHILD_FRAME_H_