Allow Conditional Mediation without autofill

[kenrb]

This is not intended for level 3.

Proposed Change

I have added an explainer to the wiki proposing a new UI mechanism and a way for sites to trigger it. The motivation is to provide conditional UI functionality that is not tied to autofill, so it can be used to sign in to an RP without necessarily being on a dedicated sign-in page.

As with existing conditional UI, the user agent would ignore the request if there are no eligible credentials for the current RP.

At Google we have been tentatively labeling this "ambient UI", reflecting that we are imagining an unobtrusive non-modal UI surface that can be ignored by the user.