PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_layer_compositor_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.
 
 #ifndef PPAPI_THUNK_PPB_LAYER_COMPOSITOR_API_H_
 #define PPAPI_THUNK_PPB_LAYER_COMPOSITOR_API_H_
 
+#include "base/memory/ref_counted.h"
 #include "ppapi/c/dev/ppb_layer_compositor_dev.h"
 
 namespace ppapi {
+
+class TrackedCallback;
+
 namespace thunk {
 
 class PPB_LayerCompositor_API {
  public:
   virtual ~PPB_LayerCompositor_API() {}
 
   virtual PP_Bool AddLayer(PP_Resource layer) = 0;
   virtual void RemoveLayer(PP_Resource layer) = 0;
   virtual void SetZIndex(PP_Resource layer, int32_t index) = 0;
   virtual void SetRect(PP_Resource layer, const PP_Rect* rect) = 0;
   virtual void SetDisplay(PP_Resource layer, PP_Bool is_displayed) = 0;
   virtual void MarkAsDirty(PP_Resource layer) = 0;
-  virtual int32_t SwapBuffers(PP_CompletionCallback callback) = 0;
+  virtual int32_t SwapBuffers(scoped_refptr<TrackedCallback> callback) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_LAYER_COMPOSITOR_API_H_