waitForMovement(mediaElement) {
    console.assert(!mediaElement.ended, 'Video should not be ended!');
    const timeGoal = mediaElement.currentTime + 1;
    return this.waitUntilPlayheadReaches(mediaElement, timeGoal);
  }
Wait for the video playhead to move forward by some meaningful delta.  The
Promise is resolved when the playhead moves or the video ends.

@param {!HTMLMediaElement} mediaElement
@return {!Promise}