PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_host_resolver_private_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_HOST_RESOLVER_PRIVATE_API_H_
 #define PPAPI_THUNK_PPB_HOST_RESOLVER_PRIVATE_API_H_
 
+#include "base/memory/ref_counted.h"
 #include "ppapi/c/private/ppb_host_resolver_private.h"
 #include "ppapi/thunk/ppapi_thunk_export.h"
 
 namespace ppapi {
+
+class TrackedCallback;
+
 namespace thunk {
 
 class PPAPI_THUNK_EXPORT PPB_HostResolver_Private_API {
  public:
   virtual ~PPB_HostResolver_Private_API() {}
 
   virtual int32_t Resolve(const char* host,
                           uint16_t port,
                           const PP_HostResolver_Private_Hint* hint,
-                          PP_CompletionCallback callback) = 0;
+                          scoped_refptr<TrackedCallback> callback) = 0;
   virtual PP_Var GetCanonicalName() = 0;
   virtual uint32_t GetSize() = 0;
   virtual bool GetNetAddress(uint32_t index,
                              PP_NetAddress_Private* addr) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_TCP_SOCKET_PRIVATE_API_H_