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/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index f716718..a15558d 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -542,6 +542,8 @@
   rtp_sender_->GetDataCounters(&rtp_stats, &rtx_stats);
 
   if (bytes_sent) {
+    // TODO(http://crbug.com/webrtc/10525): Bytes sent should only include
+    // payload bytes, not header and padding bytes.
     *bytes_sent = rtp_stats.transmitted.payload_bytes +
                   rtp_stats.transmitted.padding_bytes +
                   rtp_stats.transmitted.header_bytes +