PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_message_loop_api.h

 // 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_MESSAGE_LOOP_API_H_
 #define PPAPI_THUNK_PPB_MESSAGE_LOOP_API_H_
 
+#include "base/memory/ref_counted.h"
+#include "ppapi/c/pp_bool.h"
+#include "ppapi/c/pp_completion_callback.h"
+#include "ppapi/c/pp_stdint.h"
+
 namespace ppapi {
+
+class TrackedCallback;
+
 namespace thunk {
 
 class PPB_MessageLoop_API {
  public:
   virtual ~PPB_MessageLoop_API() {}
 
   virtual int32_t AttachToCurrentThread() = 0;
   virtual int32_t Run() = 0;
+  // Note: Most interfaces should use scoped_refptr<TrackedCallback>, in order
+  // to track callbacks and support things like blocking or optional callbacks.
+  // In this case, the callback is really just a way to pass a function pointer,
+  // and those options don't make sense.
   virtual int32_t PostWork(PP_CompletionCallback callback,
                            int64_t delay_ms) = 0;
   virtual int32_t PostQuit(PP_Bool should_destroy) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_MESSAGE_LOOP_API_H_