Whitelist new member names of RTCIceCandidatePairStats.

totalRtt and currentRtt have been renamed in the spec to
totalRoundTripTime and currentRoundTripTime:
https://w3c.github.io/webrtc-stats/#since-21-sep-2016*

Both old and new names are whitelisted until
https://codereview.webrtc.org/2576383002/ has rolled into Chromium.
When the roll is done old names will be removed.

BUG=chromium:627816

Review-Url: https://codereview.chromium.org/2577923004
Cr-Commit-Position: refs/heads/master@{#438808}

chrome/test/data/webrtc/peerconnection_getstats.js

@@ -196,6 +196,11 @@ var kRTCIceCandidatePairStats = new RTCStats_(null, {
   readable: 'boolean',
   bytesSent: 'number',
   bytesReceived: 'number',
+  totalRoundTripTime: 'number',
+  currentRoundTripTime: 'number',
+  // TODO(hbos): *Rtt have been renamed to *RoundTripTime, both listed at the
+  // moment. As soon as https://codereview.webrtc.org/2576383002/ has rolled in,
+  // remove the *Rtt ones. crbug.com/627816
   totalRtt: 'number',
   currentRtt: 'number',
   availableOutgoingBitrate: 'number',