PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_file_system_api.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 PPAPI_THUNK_PPB_FILE_SYSTEM_API_H_
 #define PPAPI_THUNK_PPB_FILE_SYSTEM_API_H_
 
 #include "ppapi/c/ppb_file_system.h"
+#include "ppapi/shared_impl/api_callback_type.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_FileSystem_API {
  public:
   virtual ~PPB_FileSystem_API() {}
 
   virtual int32_t Open(int64_t expected_size,
-                       PP_CompletionCallback callback) = 0;
+                       ApiCallbackType callback) = 0;
   virtual PP_FileSystemType GetType() = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_FILE_SYSTEM_API_H_