ertificateExpiration
   *    dictionary RTCCertificateExpiration {
   *      [EnforceRange]
   *      DOMTimeStamp expires;
   *    };
   *
   *    If this parameter is present it indicates the maximum time that
   *    the RTCCertificate is valid for relative to the current time.
   *
   *    When generateCertificate is called with an object argument,
   *    the user agent attempts to convert the object into a
   *    RTCCertificateExpiration. If this is unsuccessful, immediately
   *    return a promise 