PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_graphics_2d_api.h

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 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.
 
+#include "base/memory/ref_counted.h"
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_point.h"
 #include "ppapi/c/pp_rect.h"
 #include "ppapi/c/pp_resource.h"
 #include "ppapi/c/pp_size.h"
 
 namespace ppapi {
+
+class TrackedCallback;
+
 namespace thunk {
 
 class PPB_Graphics2D_API {
  public:
   virtual ~PPB_Graphics2D_API() {}
 
   virtual PP_Bool Describe(PP_Size* size, PP_Bool* is_always_opaque) = 0;
   virtual void PaintImageData(PP_Resource image_data,
                               const PP_Point* top_left,
                               const PP_Rect* src_rect) = 0;
   virtual void Scroll(const PP_Rect* clip_rect,
                       const PP_Point* amount) = 0;
   virtual void ReplaceContents(PP_Resource image_data) = 0;
-  virtual int32_t Flush(PP_CompletionCallback callback) = 0;
+  virtual int32_t Flush(scoped_refptr<TrackedCallback> callback) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi