Network emulation layer provides per source split for incoming stats for
endpoint. Do the same for outgoing stats per destination.
Bug: webrtc:11756
Change-Id: I2369ae8906546c27133273b1be17ce74c253c6e8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180500
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31820}