Initial webrtc and getUserMedia DOM support
This is able to reach the point where connections are properly negotiated and ready to exchange streams.
<s>The `toJSON()` stuff doesn't work yet, so most example code will need to be tweaked to manually construct JSON first before sending SDP and ICE messages over websockets. I'll add support for this soon. (This may need webidl tweaks to support `[Default]` and `toJSON()`)</s>
For some reason I haven't yet figured out, connections are one-way, Servo is able to receive streams but the other end doesn't see the streams Servo sends. I don't think this is due to https://github.com/servo/media/issues/191, but that bug is making it harder to test.
This implementation simply drops streams that it receives, without connecting them up to any output elements, since servo-media-player doesn't yet have mediastream support.
Since servo can neither effectively send nor receive streams this implementation isn't useful yet, however it is getting large and I figured I'd get it reviewed and landed early as a base.
r? @jdm
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