chromium/chrome/test/data/extensions/api_test/app_background_page/bg_open/bg_open_bg.html

This is the background contents within the hosted app. When this page loads
it opens a popup window - the extension background page in test.html will
detect this and cause the test to pass.

<script>
  window.onload = function() {
    window.open("popup.html", "_blank");
  };
</script>