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/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 9b1b6d0..bb13c20 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -762,7 +762,11 @@
       verifier.TestMemberIsUndefined(outbound_stream.qp_sum);
     }
     verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.packets_sent);
+    verifier.TestMemberIsNonNegative<uint64_t>(
+        outbound_stream.retransmitted_packets_sent);
     verifier.TestMemberIsNonNegative<uint64_t>(outbound_stream.bytes_sent);
+    verifier.TestMemberIsNonNegative<uint64_t>(
+        outbound_stream.retransmitted_bytes_sent);
     verifier.TestMemberIsUndefined(outbound_stream.target_bitrate);
     if (outbound_stream.media_type.is_defined() &&
         *outbound_stream.media_type == "video") {