static parseTREX(reader) {
    reader.skip(4); // Skip "track_ID"
    reader.skip(4); // Skip "default_sample_description_index"
    const defaultSampleDuration = reader.readUint32();
    const defaultSampleSize = reader.readUint32();

    return {
      defaultSampleDuration,
      defaultSampleSize,
    };
  }
Parses a TREX Box.
@param {!shaka.util.DataViewReader} reader
@return {!shaka.util.ParsedTREXBox}