The lock in TransportFeedbackProxy could cause a dead-lock if audio is
included in transport feedback messages, and necessitated a revert:
https://webrtc-review.googlesource.com/c/src/+/178100
This CL removes that lock and in fact the entire TransportFeedbackProxy
class, and instead sets the observer at construction time.
We therefore don't need to guard the observer pointer anymore.
For further context, see also internal bug b/153893626
Bug: webrtc:10809
Change-Id: I79b08d8d0e587a59736b383c3596a26836c33d2e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178207
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31583}
2b4d2f3561