This adds a method to allow VideoLayersAllocation to be sent using the header extension RtpVideoLayersAllocationExtension. Bug: webrtc:12000 Change-Id: Iafdc1e16911c57ca55d7cc0559a0b45774211e92 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187495 Commit-Queue: Per Kjellander <perkj@webrtc.org> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32397}