[webrtc-stats] Add metric for ICE reroutes (#474)

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

== Add metric for ICE reroutes ==
High number of ICE restarts negatively correlate with bad network/quality, and would be a useful metric to have.

An ICE reroute is when the ICE candidate pair changes. I propose adding:

> unsigned long RTCTransportStats.selectedCandidatePairChanges:
> The number of times that the selected candidate pair of this transport has changed.


Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/474 using your GitHub account