equals(e.data['containerSrc'], '');
      t.done();
    }));
    const observingFrame = document.createElement('iframe');
    observingFrame.id = 'observing-iframe-id';
    observingFrame.name = 'observing-iframe-name';
    document.body.appendChild(observingFrame);
    observingFrame.src = 'resources/subframe-observing-longtask.html'

    observingFrame.onload = () => {
      /* Create a cross-origin iframe that generates a long task. */
      const longtaskFrame = document.createElement('iframe');
      l