constructor() {
    /** @private {?MediaStreamSource} set by updateSource*/
    this.source_ = null;
    /** @private {?FrameTransform} set by updateTransform */
    this.frameTransform_ = null;
    /** @private {?MediaStreamSink} set by updateSink */
    this.sink_ = null;
    /** @private {!AbortController} may used to stop all processing */
    this.abortController_ = new AbortController();
    /**
     * @private {?MediaStream} set in maybeStartPipeline_ after all of source_,
     *     frameTransform_, and sink_ are set
     */
    this.processedStream_ = null;
  }
Assembles a MediaStreamSource, FrameTransform, and MediaStreamSink together.