<html>
  <body>
    <h3>test page</h3>
    <iframe src="about:blank"></iframe>
    <script>
      "use strict";

      window.onmessage = function(evt) {
        if (evt.data === "updated-iframe-url") {
          window.postMessage("frame-updated", "*");
        }
      };
      window.onload = function() {
        document.querySelector("iframe").setAttribute("src", "context_tabs_onUpdated_iframe.html");
      };
    </script>
  </body>
</html>
