Rename RTCOutboundRTPStreamStats *_rtt members to *_round_trip_time.

The spec renamed these recently:
https://w3c.github.io/webrtc-stats/#since-21-sep-2016*

BUG=chromium:627816

Review-Url: https://codereview.webrtc.org/2576383002
Cr-Commit-Position: refs/heads/master@{#15630}
diff --git a/webrtc/api/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index 04797e9..429f93b 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -134,10 +134,10 @@
   RTCStatsMember<uint64_t> bytes_sent;
   RTCStatsMember<uint64_t> bytes_received;
   // TODO(hbos): Not collected by |RTCStatsCollector|. crbug.com/633550
-  RTCStatsMember<double> total_rtt;
+  RTCStatsMember<double> total_round_trip_time;
   // TODO(hbos): Collected by |RTCStatsCollector| but different than the spec.
   // crbug.com/633550
-  RTCStatsMember<double> current_rtt;
+  RTCStatsMember<double> current_round_trip_time;
   // TODO(hbos): Not collected by |RTCStatsCollector|. crbug.com/633550
   RTCStatsMember<double> available_outgoing_bitrate;
   // TODO(hbos): Not collected by |RTCStatsCollector|. crbug.com/633550