PPAPI: Implement synchronous postMessage

ppapi/api/ppb_messaging.idl

Index: ppapi/api/ppb_messaging.idl
diff --git a/ppapi/api/ppb_messaging.idl b/ppapi/api/ppb_messaging.idl
index dc50788e26db2c0fbfbd9824e260c4ae76f5a8d2..5100f5f5ed879ef6e764cbcaab7367276eb00750 100644
--- a/ppapi/api/ppb_messaging.idl
+++ b/ppapi/api/ppb_messaging.idl
@@ -86,9 +86,6 @@ interface PPB_Messaging {
   void PostMessage([in] PP_Instance instance, [in] PP_Var message);
 
   /**
-   * <strong>Note:</strong> This function is not yet implemented. Please use
-   * PPB_Messaging_1_0.
-   *
    * Registers a handler for receiving messages from JavaScript. If a handler
    * is registered this way, it will replace PPP_Messaging, and all messages
    * sent from JavaScript via postMessage and postMessageAndAwaitResponse will
@@ -99,6 +96,12 @@ interface PPB_Messaging {
    * <code>message_loop</code> is attached, when <code>message_loop</code> is
    * run. It is illegal to pass the main thread message loop;
    * RegisterMessageHandler will return PP_ERROR_WRONG_THREAD in that case.
+   * If you quit <code>message_loop</code> before calling Unregister(),
+   * the browser will not be able to call functions in the plugin's message
+   * handler any more. That could mean missing some messages or could cause a
+   * leak if you depend on Destroy() to free hander data. So you should,
+   * whenever possible, Unregister() the handler prior to quitting its event
+   * loop.
    *
    * Attempting to register a message handler when one is already registered
    * will cause the current MessageHandler to be unregistered and replaced. In
@@ -123,9 +126,6 @@ interface PPB_Messaging {
                                  [in] PPP_MessageHandler handler,
                                  [in] PP_Resource message_loop);
   /**
-   * <strong>Note:</strong> This function is not yet implemented. Please use
-   * PPB_Messaging_1_0.
-   *
    * Unregisters the current message handler for <code>instance</code> if one
    * is registered. After this call, the message handler (if one was
    * registered) will have "Destroy" called on it and will receive no further