addExistingStream(id) {
    const stream = this.manifest_.findExistingStream_(id);
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    goog.asserts.assert(stream, 'Must list an existing stream ID');
    if (stream.type == ContentType.AUDIO) {
      this.audio = stream;
    } else if (stream.type == ContentType.VIDEO) {
      this.video = stream;
    } else {
      goog.asserts.assert(false, 'Cannot add existing text streams');
    }
  }
Adds an existing stream to the current variant.

@param {number} id