Implement RTCOutboundRtpStreamStats.retransmitted[Bytes/Packets]Sent.

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-retransmittedpacketssent

These are already existed in StreamDataCounters. This CL takes care of
the plumbing of these values to the standard stats collector.

TBR=solenberg@webrtc.org

Bug: webrtc:10447
Change-Id: I27d6c3ee3ab627d306303e6ee67e586ddf31cc81
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/132012
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27663}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index 81ec084..4ddfa93 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -442,7 +442,9 @@
   ~RTCOutboundRTPStreamStats() override;
 
   RTCStatsMember<uint32_t> packets_sent;
+  RTCStatsMember<uint64_t> retransmitted_packets_sent;
   RTCStatsMember<uint64_t> bytes_sent;
+  RTCStatsMember<uint64_t> retransmitted_bytes_sent;
   // TODO(hbos): Collect and populate this value. https://bugs.webrtc.org/7066
   RTCStatsMember<double> target_bitrate;
   RTCStatsMember<uint32_t> frames_encoded;