enable method
disableRejectionDelay Allows callers to disable the promise rejection delay that would
normally happen, if this is unimportant to what's being tested.
(step 4 of https://fedidcg.github.io/FedCM/#browser-api-rp-sign-in)
Implementation
Future<void> enable({bool? disableRejectionDelay}) async {
  await _client.send('FedCm.enable', {
    if (disableRejectionDelay != null)
      'disableRejectionDelay': disableRejectionDelay,
  });
}