chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/navigating-across-documents/multiple-globals/resources/target.js

window.onload = function() {
    let testWindow;
    if (opener) {
      testWindow = opener.top;
    } else {
      testWindow = top;
    }
    testWindow.postMessage(
        {location: location.href, referrer: document.referrer},
        "*");
}