Update webrtc stats bridge for modern Chrome/Firefox APIs

Switch to the promise-based getStats, and use the standardized
property names where available, with fallback to handle Chrome,
which hasn't fully switched yet.

The stats now only track the outgoing audio stream, as a way to
indicate quality, since the user can't hear themselves. This
might be revisited later; it would require tracking the stats
separately for incoming vs. outgoing and e.g. returning whichever
is worse.