Adds support for Vp8 simulcast. Bug: webrtc:12000 Change-Id: Ib24fd0542642b023ec35f7a7bdc4880d72365edf Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187341 Commit-Queue: Per Kjellander <perkj@webrtc.org> Reviewed-by: Erik Språng <sprang@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32416}