[webrtc-stats] RTCOutboundRtpStreamStats.retransmitted[Bytes/Packets]Sent (#418)

henbos has just created a new issue for https://github.com/w3c/webrtc-stats:

== RTCOutboundRtpStreamStats.retransmitted[Bytes/Packets]Sent ==
In the case of RTX this happens on separate SSRCs and this separate RTCOutboundRtpStreamStats with separate bytesSent counters, but for good ol' fashioned retransmissions we don't have counters. The existing [bytesSent](https://w3c.github.io/webrtc-stats/#dom-rtcsentrtpstreamstats-bytessent) is "Total number of bytes sent for this SSRC".

Being able to tell how many bytes/packets were retransmission is useful for network diagnostic, and being able to tell how much of the bytes/packets was not retransmitted is useful for comparing that total with the expected output from the encoder (finding out of the encoder overshoots etc).

**Proposal**
Add retransmittedBytesSent and retransmittedPacketsSent as the "Total number of retransmitted bytes/packets sent for this SSRC."


Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/418 using your GitHub account