static getVideoElement_() {
    const videos = document.getElementsByTagName('video');
    const video = videos.length ? videos[0] : document.createElement('video');
    return /** @type {!HTMLVideoElement} */(video);
  }
For some of this polyfill's implementation, we need to query a video
element.  But for some embedded systems, it is memory-expensive to create
multiple video elements.  Therefore, we check the document to see if we can
borrow one to query before we fall back to creating one temporarily.

@return {!HTMLVideoElement}
@private