RTCOutboundRTPStreamStats.roundTripTime: Only report non-negative values.

Underlying stats gatherers may otherwise default it to -1.

BUG=chromium:669877, chromium:627816

Review-Url: https://codereview.webrtc.org/2562703007
Cr-Commit-Position: refs/heads/master@{#15625}
diff --git a/webrtc/api/rtcstatscollector.cc b/webrtc/api/rtcstatscollector.cc
index 5289c48..f5d3e5f 100644
--- a/webrtc/api/rtcstatscollector.cc
+++ b/webrtc/api/rtcstatscollector.cc
@@ -197,8 +197,10 @@
       static_cast<uint32_t>(media_sender_info.packets_sent);
   outbound_stats->bytes_sent =
       static_cast<uint64_t>(media_sender_info.bytes_sent);
-  outbound_stats->round_trip_time =
-      static_cast<double>(media_sender_info.rtt_ms) / rtc::kNumMillisecsPerSec;
+  if (media_sender_info.rtt_ms >= 0) {
+    outbound_stats->round_trip_time = static_cast<double>(
+        media_sender_info.rtt_ms) / rtc::kNumMillisecsPerSec;
+  }
 }
 
 void SetOutboundRTPStreamStatsFromVoiceSenderInfo(