To allow various VideoBitrateAllocators to use propagated rather than global field trials Bug: webrtc:42220378 Change-Id: I52816628169a54b18a4405d84fee69b101f92f72 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/349920 Reviewed-by: Erik Språng <sprang@webrtc.org> Reviewed-by: Philip Eliasson <philipel@webrtc.org> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Cr-Commit-Position: refs/heads/main@{#42288}