getRoundTripTime() {
    const key = shaka.util.CmsdManager.KEYS_.ROUND_TRIP_TIME;
    if (!this.config_.enabled || !this.dynamicParams_ ||
        !this.dynamicParams_.has(key)) {
      return null;
    }
    return /** @type {number} */(this.dynamicParams_.get(key));
  }
Returns the RTT in milliseconds. If there is no RTT or it's not enabled,
it returns null.

@return {?number}
@export