setupPreferredTextOnSrc_() {
    const preferredTextLanguage = this.config_.preferredTextLanguage;

    // If the user has not selected a preference, the browser preference is
    // left.
    if (preferredTextLanguage == '') {
      return;
    }

    const preferForcedSubs = this.config_.preferForcedSubs;
    const preferredTextRole = this.config_.preferredTextRole;

    this.selectTextLanguage(preferredTextLanguage, preferredTextRole,
        preferForcedSubs);
  }
This method setup the preferred text using src=.

@private