Support spec-compliant getStats() only

"Firefox has supported getStats to the spec for a couple of years now, and Chrome Canary finally has a spec-compliant version, so it’s a good time to check in how to write code that (soon!) will work across browsers."

https://blog.mozilla.org/webrtc/fiddle-week-spec-compliant-getstats/

Once Canary progresses to stable (in ~12 weeks), we should consider only supporting the promises version. Other implementations can just assume no getStats exists, which only means that `remoteAddress`, `remotePort` properties will be `undefined`. No big deal.