parseByteRange_(previousReference, byterange) {
    let startByte = 0;
    let endByte = null;
    // If BYTERANGE is not specified, the segment consists of the entire
    // resource.
    if (byterange) {
      const blocks = byterange.split('@');
      const byteLength = Number(blocks[0]);
      if (blocks[1]) {
        startByte = Number(blocks[1]);
      } else {
        goog.asserts.assert(previousReference,
            'Cannot refer back to previous HLS segment!');
        startByte = previousReference.endByte + 1;
      }
      endByte = startByte + byteLength - 1;
    }
    return [startByte, endByte];
  }
Parse the startByte and endByte.
@param {shaka.media.SegmentReference} previousReference
@param {?string} byterange
@return {!Array<number>} An array with the start byte and end byte.
@private