[webrtc-pc] Mark multiple DTLS certificates at risk (#2498)

dontcallmedom has just created a new issue for https://github.com/w3c/webrtc-pc:

== Mark multiple DTLS certificates at risk ==
None of the implementations tested in WPT pass the [test of supporting multiple certificates](https://github.com/web-platform-tests/wpt/blob/master/webrtc/RTCCertificate.html#L237) when creating (or configuring) an `RTCPeerConnection` object, as [required by the spec](https://w3c.github.io/webrtc-pc/webrtc.html#dom-rtcconfiguration-certificates).

The relevant [Chromium bug](https://bugs.chromium.org/p/webrtc/issues/detail?id=5934) hasn't seen much activity recently, and the relevant [Firefox bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1531880) is marked P3.

Should this feature be marked at risk?

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2498 using your GitHub account