Do not expose unknown usernameFragment to stats

[fippo]

https://github.com/webtorrent/webtorrent/issues/288#issuecomment-2433534469 points out that the remote ICE usernameFragment (added in https://github.com/w3c/webrtc-stats/pull/611) can act as a sidechannel (and while it is authenticated, it lacks encryption).

In theory one can skip signaling in one direction that way. We should only expose the remote usernameFragment on prflx candidates if it is known by signaling.

[alvestrand]

I think this is an obvious fix, and should be marked "ready for PR".

[fippo]

Do we need to put it on the WG agenda?

[dontcallmedom-bot]

This issue had an associated resolution in WebRTC February 2025 meeting – (Issue #789 Do not expose unknown usernameFragment to stats.):

RESOLUTION: Support, review the PR again.