dtlsRole/iceRole "unknown" is awkward

[fippo]

https://w3c.github.io/webrtc-stats/#dom-rtctransportstats-dtlsrole
unknown means "we haven't figured that out yet - determined by the SDP negotiation and the respective setup attributes.
Same for https://www.w3.org/TR/webrtc/#rtcicerole

Leaving it undefined until it is known seems more aligned with
https://w3c.github.io/webrtc-stats/#guidelines-for-implementing-stats-objects

When a feature is an instantaneous measure and it has not yet been sampled, it shall not exist.

Practical impact and breakage risk is low. This only happens in the following case:
https://jsfiddle.net/fippo/rpvcf1jL/1/
wherein there offerer uses "actpass" which can turn into either.

See also #804

[docfaraday]

I agree completely. This is what nullable is for on interfaces, and stats dictionaries don't even need a webidl modification for this.