Specify what happens when a WebAuthn request is issued while there is another one in progress

[nsatragno]

I noticed the specification does not say what happens when a WebAuthn request is issued while another one is still pending. On Chrome, this results in an error.

WebAuthn credentials may not be the only credentials in the browser accessed through the Credential Management API that result in some UI being shown. This should probably be addressed at the Credential Management level then, allowing a single pending request per frame. If the RP decides to cancel a request (e.g. a conditional ui request) to issue another, they can use the abort controller.

[nsatragno]

We'll be tracking this on credential management: w3c/webappsec-credential-management#206

From the w3c call, we might want to add some sort of guidance to RPs on WebAuthn, especially with respect to conditional UI requests.