constructor() {
    /**
     * @private @const {!VideoMirrorHelper} manages displaying the video stream
     *     in the page
     */
    this.videoMirrorHelper_ = new VideoMirrorHelper();
    /** @private {?MediaStream} camera stream, initialized in getMediaStream */
    this.stream_ = null;
    /** @private {string} */
    this.debugPath_ = '<unknown>';
  }
Opens the device's camera with getUserMedia.
@implements {MediaStreamSource} in pipeline.js