RTCIceCandidatePairStats.[total/current]RoundTripTime collected.

Collected in accordance with the spec:
https://w3c.github.io/webrtc-stats/#candidatepair-dict*

totalRoundTripTime is collected as the sum of rtt measurements, it was
previously not collected.
currentRoundTripTime is collected as the latest rtt measurement, it
was previously collected as a smoothed value, which was incorrect.

Connection is updated to collect these values which are surfaced
through ConnectionInfo.

BUG=webrtc:7062, webrtc:7204

Review-Url: https://codereview.webrtc.org/2719523002
Cr-Commit-Position: refs/heads/master@{#16905}