chromium/third_party/blink/web_tests/http/tests/security/mixedContent/resources/frame-with-about-blank-frame.html

<script>
    var numFramesLoaded = 0;
    function loadedFrame() {
        numFramesLoaded++;
        if (numFramesLoaded === 2 && window.opener)
            window.opener.postMessage('done', '*');
    }
    window.addEventListener('load', function () {
        if (window.opener)
            window.opener.postMessage('done', '*');
    });
</script>
<iframe src="about:blank" onload='loadedFrame();'></iframe><br>
<iframe src="" onload='loadedFrame();'></iframe>