static clearSourceFromVideo(video) {
    let result = false;
    const sources = video.getElementsByTagName('source');
    for (let i = sources.length - 1; i >= 0; --i) {
      video.removeChild(sources[i]);
      result = true;
    }
    if (video.src) {
      video.removeAttribute('src');
      result = true;
    }
    return result;
  }
Remove all source elements and src attribute from a video element.
Returns true if any change was made.
@param {!HTMLMediaElement} video
@export
@return {boolean}