chooseTextStream_() {
    const subset = shaka.util.StreamUtils.filterStreamsByLanguageAndRole(
        this.manifest_.textStreams,
        this.currentTextLanguage_,
        this.currentTextRole_,
        this.currentTextForced_);
    return subset[0] || null;
  }
Choose a text stream from all possible text streams while taking into
account user preference.

@return {?shaka.extern.Stream}
@private