third_party/blink/web_tests/http/tests/security/mixedContent/resources/frame-with-data-url-frame.html

<script>
function frameLoaded() {
  if (window.opener)
    window.opener.postMessage('done', '*');
}
</script>
<iframe onload="frameLoaded()"
        src="data:text/html,This is a boring HTML document."></iframe>