parseMasterVariables_(tags) {
    const queryParams = new goog.Uri(this.masterPlaylistUri_).getQueryData();
    for (const variableTag of tags) {
      const name = variableTag.getAttributeValue('NAME');
      const value = variableTag.getAttributeValue('VALUE');
      const queryParam = variableTag.getAttributeValue('QUERYPARAM');
      if (name && value) {
        if (!this.globalVariables_.has(name)) {
          this.globalVariables_.set(name, value);
        }
      }
      if (queryParam) {
        const queryParamValue = queryParams.get(queryParam)[0];
        if (queryParamValue && !this.globalVariables_.has(queryParamValue)) {
          this.globalVariables_.set(queryParam, queryParamValue);
        }
      }
    }
  }
Get the variables of each variant tag, and store in a map.
@param {!Array<!shaka.hls.Tag>} tags Variant tags from the playlist.
@private