making RTCOutboundRtpStreamStats inherit from RTCSentRtpStreamStats as defined in https://www.w3.org/TR/webrtc-stats/#outboundrtpstats-dict* This removes the duplicated definitions of packetsSent and bytesSent. BUG=webrtc:14948 Change-Id: I184998b65d59dbd0d1288733d55d8a884e6de970 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/295660 Reviewed-by: Harald Alvestrand <hta@webrtc.org> Commit-Queue: Philipp Hancke <phancke@microsoft.com> Reviewed-by: Henrik Boström <hbos@webrtc.org> Cr-Commit-Position: refs/heads/main@{#39481}