[webrtc-stats] Expose SCTP stats in RTCSctpTransportStats (#548)

shacharz has just created a new issue for https://github.com/w3c/webrtc-stats:

== Expose SCTP stats in RTCSctpTransportStats ==
Applications that use data channels intensively would find it useful to have data from SCTP layer:
- **congestion window size** (spinfo_cwnd)
- **receiver window size** (sstat_rwnd)
- **MTU** (spinfo_mtu)
- **packet loss ratio** (not exposed directly in sctp as far as I can tell, but might be able to calculate from sstat_unackdata)

For example:
- congestion, receiver, MTU and rtt together can predict the throughput of the channel, and help prioritize different channels.
- packet loss can help identifying a problem somewhere in the network path.

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/548 using your GitHub account