chromium/android_webview/test/data/post_message_array_buffer_reply.html

<html>

<head>
  <title>reply</title>
  <script>
    myObject.onmessage = function (event) {
      try {
        myObject.postMessage(event.data);
      } catch (e) {
        // ArrayBuffer may not supported, echo error back.
        myObject.postMessage(e.toString());
      }
    };
    myObject.postMessage("Hello");
  </script>
</head>

<body>
  <div>post message waiting for reply</div>
</body>

</html>