chromium/chrome/test/data/extensions/api_test/automation/sites/iframenav/iframe-top.html

<!DOCTYPE html>
<html>

<div>
  <iframe id="frame1" src="iframe-button.html"></iframe>
</div>

<script>
  function doNavFunc() {
    let iframe = document.getElementById("frame1");
    iframe.src="iframe-button2.html";
  }
</script>

</html>