chooseVariant_() {
    if (this.updateAbrManagerVariants_()) {
      return this.abrManager_.chooseVariant();
    } else {
      return null;
    }
  }
Chooses a variant from all possible variants while taking into account
restrictions, preferences, and ABR.

On error, this dispatches an error event and returns null.

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