ppapi: Cleanup class/struct forward declarations

ppapi/cpp/private/ext_crx_file_system_private.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 PPAPI_CPP_PRIVATE_EXTENSION_CRX_FILE_SYSTEM_PRIVATE_H_
 #define PPAPI_CPP_PRIVATE_EXTENSION_CRX_FILE_SYSTEM_PRIVATE_H_
 
 #include <stdint.h>
 
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/cpp/completion_callback.h"
 #include "ppapi/cpp/file_system.h"
 #include "ppapi/cpp/instance_handle.h"
 
 namespace pp {
 
-class CompletionCallback;
-
 class ExtCrxFileSystemPrivate {
  public:
   ExtCrxFileSystemPrivate();
   explicit ExtCrxFileSystemPrivate(const InstanceHandle& instance);
   virtual ~ExtCrxFileSystemPrivate();
 
   int32_t Open(const CompletionCallbackWithOutput<pp::FileSystem>& cc);
 
  private:
   PP_Instance instance_;
 };
 
 }  // namespace pp
 
 #endif  // PPAPI_CPP_PRIVATE_EXTENSION_CRX_FILE_SYSTEM_PRIVATE_H_