constructor() {
    /** @private {boolean} */
    this.visibility_ = false;
    /** @private {?MediaStream} the stream to display */
    this.stream_ = null;
    /**
     * @private {?HTMLVideoElement} video element mirroring the camera stream.
     *    Set if visibility_ is true and stream_ is set.
     */
    this.video_ = null;
    /** @private {string} */
    this.debugPath_ = '<unknown>';
  }
Helper to display a MediaStream in an HTMLVideoElement, based on the
visibility setting.