We currently end up ignore incoming mDNS addresses. After they are resolved, we do not send the updated candidate to nICEr here [1] but instead pass in the original candidate with the obfuscated address.

[1] https://searchfox.org/mozilla-central/rev/0da35261b6789eec65476dbdd4913df6e235af6d/media/webrtc/signaling/src/peerconnection/MediaTransportHandler.cpp#1415

This must have been broken while addressing review comments in Bug 1548841,
the first commit has it right and the last commit in the series breaks it.
Since we don't yet generate mDNS addresses in Firefox, the unit tests would not
catch this problem, and interoperability with other browsers would continue to
work due to peer reflex candidates.

We will have test coverage for this once we land generating our own mDNS
candidates. ICE will fail in the mochitests if we don't support resolving
mDNS addresses properly, which is how I discovered this bug.

https://hg.mozilla.org/mozilla-central/rev/682ce4a4a4de