Cleanup: Remove more forward declarations in various chrome/browser directories.

chrome/browser/plugin_observer.h

 // Copyright (c) 2011 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_PLUGIN_OBSERVER_H_
 #define CHROME_BROWSER_PLUGIN_OBSERVER_H_
 #pragma once
 
 #include "content/browser/tab_contents/tab_contents_observer.h"
 
-class FilePath;
 class GURL;
 class InfoBarDelegate;
-class PluginInstallerInfoBarDelegate;
 class TabContentsWrapper;
 
 class PluginObserver : public TabContentsObserver {
  public:
   explicit PluginObserver(TabContentsWrapper* tab_contents);
   virtual ~PluginObserver();
 
   // IPC::Channel::Listener implementation.
   virtual bool OnMessageReceived(const IPC::Message& message);
 
  private:
   void OnBlockedOutdatedPlugin(const string16& name, const GURL& update_url);
 
   TabContentsWrapper* tab_contents_;
   scoped_ptr<InfoBarDelegate> plugin_installer_;  // Lazily created.
 
   DISALLOW_COPY_AND_ASSIGN(PluginObserver);
 };
 
 #endif  // CHROME_BROWSER_PLUGIN_OBSERVER_H_