This is a follow-up to r34019. It adds checks for when padding can be sent before media - and how timestamps are set on RTX padding. Bug: webrtc:11340 Change-Id: I46fbd3c3eff9e308b5c65220718df749f2d9c46b Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/219162 Commit-Queue: Erik Språng <sprang@webrtc.org> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/master@{#34041}