Allow showing the widget UI for logged-out users

[cbiesinger]

Issue w3c-fedid/FedCM#568 is requesting a way to show a widget UI onload for users who are not logged in to the IDP.

Filing this issue to track the feature request. This seems perfectly valid; we'd have to work through some UI issues. If we have the more quiet omnibox UI that would be a perfect match but maybe we can do something before that.

Implementing the conditional UI (autofill dropdown) might also address the use case.