This CL fixes two things related to the (not yet active) new
PacedSender code path:
1. Make sure BWE header extensions are properly populated for all
padding packets.
2. When generating padding, don't hold the RtpSender critsect when
accessing the RtpPacketHistory as this may lead to a lock order
inversion.
Bug: webrtc:10633
Change-Id: I8650fbf5dafddbeae61837d2137338163e1c48ce
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145723
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28613}