static getVariantSummaryString_(variant) {
    const summaries = [];
    if (variant.audio) {
      summaries.push(shaka.util.StreamUtils.getStreamSummaryString_(
          variant.audio));
    }
    if (variant.video) {
      summaries.push(shaka.util.StreamUtils.getStreamSummaryString_(
          variant.video));
    }
    return summaries.join(', ');
  }
Returns a string of a variant, with the attribute values of its audio
and/or video streams for log printing.
@param {shaka.extern.Variant} variant
@return {string}
@private