PPAPI: Add C++ wrapper for MessageHandler stuff.

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 14 matching lines @@
  */
 [no_interface_string]
 interface PPP_MessageHandler {
   /**
    * Invoked as a result of JavaScript invoking postMessage() 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.
+   * RegisterMessageHandler().
    * @param[in] message A copy of the parameter that JavaScript provided to
    * postMessage().
    */
   void HandleMessage([in] PP_Instance instance,
                      [inout] mem_t user_data,
                      [constptr_in] PP_Var message);
   /**
    * Invoked as a result of JavaScript invoking postMessageAndAwaitResponse()
    * on the plugin's DOM element.
    *
+   * NOTE: JavaScript execution is blocked during the duration of this call.
+   * Hence, the plugin should respond as quickly as possible. For this reason,
+   * blocking completion callbacks are disallowed while handling a blocking
+   * message.
+   *
    * @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.
+   * RegisterMessageHandler().
    * @param[in] message is a copy of the parameter that JavaScript provided
-   * to postMessageAndAwaitResponse.
+   * to postMessageAndAwaitResponse().
    * @param[out] response will be copied to a JavaScript object which is
-   * returned as the result of postMessageAndAwaitResponse to the invoking
+   * returned as the result of postMessageAndAwaitResponse() to the invoking
      JavaScript.
    */
   void HandleBlockingMessage([in] PP_Instance instance,
                              [inout] mem_t user_data,
                              [constptr_in] PP_Var message,
                              [out] PP_Var response);
   /**
    * Invoked when the handler object is no longer needed. After this, no more
    * calls will be made which pass this same value for <code>instance</code>
    * 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);
 };