setStreamPropertiesImpl_(type, offset, appendWindowEnd, sequenceMode) {
    if (!this.segments[type]) {
      throw new Error('unexpected type');
    }
    if (!sequenceMode) {
      this.timestampOffsets_[type] = offset;
    }
    // Don't use |appendWindowEnd|.
    return Promise.resolve();
  }
@param {string} type
@param {number} offset
@param {number} appendWindowEnd
@param {boolean} sequenceMode
@return {!Promise}
@private