Don't send PP_Vars/V8 values with cycles across PostMessage

ppapi/api/ppb_messaging.idl

 /* 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.
  */
 
 /**
  * This file defines the <code>PPB_Messaging</code> interface implemented
  * by the browser for sending messages to DOM elements associated with a
  * specific module instance.
  */
@@ skipping 18 matching lines @@
    * @param[in] instance A <code>PP_Instance</code> identifying one instance
    * of a module.
    * @param[in] message A <code>PP_Var</code> containing the data to be sent to
    * JavaScript.
    * <code>message</code> can be any <code>PP_Var</code> type except
    * <code>PP_VARTYPE_OBJECT</code>. Array/Dictionary types are supported from
    * Chrome M29 onward. All var types are copied when passing them to
    * JavaScript.
    *
    * When passing array or dictionary <code>PP_Var</code>s, the entire reference
-   * graph will be converted and transferred, including reference cycles if they
-   * exist.
+   * graph will be converted and transferred. If the reference graph has cycles,
+   * an undefined var will be sent instead and an error will be logged to the

[dmichael (off chromium), 2013/06/04 16:59:10]

Is this the behavior that was discussed in the meeting last week? I would have
assumed we would send no message to the other side (But log an error).

[raymes, 2013/06/04 19:36:06]

We discussed both and there wasn't a clear decision. I think both make sense (in
one way it's nice for an application to get notified if the var is ignored). But
I don't mind either way. I changed it to drop the var.

[dmichael (off chromium), 2013/06/05 17:05:47]

Yeah, I could see both sides. Dropping it seems less surprising to me, I guess.
I think passing a message with unexpected contents might be more likely to
result in weird/bad behavior to a user.

Hopefully the log message provides enough notification to the developer and/or
user.

+   * console.
    *
    * Listeners for message events in JavaScript code will receive an object
    * conforming to the HTML 5 <code>MessageEvent</code> interface.
    * Specifically, the value of message will be contained as a property called
    *  data in the received <code>MessageEvent</code>.
    *
    * This messaging system is similar to the system used for listening for
    * messages from Web Workers. Refer to
    * <code>http://www.whatwg.org/specs/web-workers/current-work/</code> for
    * further information.
@@ skipping 26 matching lines @@
    *  ppb_messaging_interface->PostMessage(instance, hello_var); // Copies var.
    *  ppb_var_interface->Release(hello_var);
    *
    * @endcode
    *
    * The browser will pop-up an alert saying "Hello world!"
    */
   void PostMessage([in] PP_Instance instance, [in] PP_Var message);
 };