Bump snowflake/go-webrtc again for trac 21312

This is a followup to #25449 (moved).

We discovered additional crashes having to do with the go-webrtc binding in #21312 (moved). Although we only saw the crashes in the proxy and server components, not the client, the attached patch applies the same synchronization fixes to the client.

I did a testbuild and put the results here (I'm using it right now to file this ticket): https://people.torproject.org/~dcf/pt-bundle/snowflake/20180321-8.0a4-4a5889af2891/