PPAPI: Implement synchronous postMessage

ppapi/c/ppp_message_handler.h

 /* Copyright 2014 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.
  */
 
-/* From ppp_message_handler.idl modified Fri May 30 15:49:17 2014. */
+/* From ppp_message_handler.idl modified Tue Jun  3 16:50:26 2014. */
 
 #ifndef PPAPI_C_PPP_MESSAGE_HANDLER_H_
 #define PPAPI_C_PPP_MESSAGE_HANDLER_H_
 
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_macros.h"
 #include "ppapi/c/pp_stdint.h"
 #include "ppapi/c/pp_var.h"
 
@@ skipping 25 matching lines @@
    * DOM element.
    *
    * @param[in] instance A <code>PP_Instance</code> identifying one instance
    * of a module.
    * @param[in] user_data is the same pointer which was provided by a call to
    * RegisterMessageHandler.
    * @param[in] message A copy of the parameter that JavaScript provided to
    * postMessage().
    */
   void (*HandleMessage)(PP_Instance instance,
-                        const void* user_data,
+                        void* user_data,
                         struct PP_Var message);
   /**
    * Invoked as a result of JavaScript invoking postMessageAndAwaitResponse()
    * on the plugin's DOM element.
    *
    * @param[in] instance A <code>PP_Instance</code> identifying one instance
    * of a module.
    * @param[in] user_data is the same pointer which was provided by a call to
    * RegisterMessageHandler.
    * @param[in] message is a copy of the parameter that JavaScript provided
@@ skipping 15 matching lines @@
    * RegisterMessageHandler.
    */
   void (*Destroy)(PP_Instance instance, void* user_data);
 };
 /**
  * @}
  */
 
 #endif  /* PPAPI_C_PPP_MESSAGE_HANDLER_H_ */