Move RtpVideoSender's worker-only state off mutex_

transport_overhead_bytes_per_packet_ and loss_mask_vector_ were guarded
by mutex_, and the bitrate members weren't annotated, even though all of
them are only accessed on the transport queue. Guard them with
transport_checker_ instead, and DCHECK that the methods called on the
transport queue run there.

OnPacketFeedbackVector() no longer takes mutex_. OnBitrateUpdated()
still does, for fec_allowed_, which is set on the encoder queue.

Bug: webrtc:42223727
Change-Id: Id24ed32d25e960fe1bcdadd0fe2f47278884d760
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506162
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48784}