PPAPI: Implement synchronous postMessage

ppapi/api/ppp_message_handler.idl

 /* 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.
  */
 
 /**
  * This file defines the <code>PPP_MessageHandler</code> interface that plugins
  * can implement and register using PPB_Messaging::RegisterMessageHandler in
  * order to handle messages sent from JavaScript via postMessage() or
  * postMessageAndAwaitResponse().
@@ skipping 19 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([in] PP_Instance instance,
-                     [in] mem_t user_data,
+                     [inout] mem_t user_data,
                      [in] 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 10 matching lines @@
    * and <code>user_data</code>.
    *
    * @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.
    */
   void Destroy([in] PP_Instance instance, [inout] mem_t user_data);
 };