Always report pacer queue size in ProcessInterval

Remove add_pacing_to_cwin_ member from RtpTransportControllerSend
and populate msg.pacer_queue in ProcessInterval unconditionally.

Safety justification:
1. ProcessInterval::pacer_queue is an std::optional<DataSize>.
2. For GoogCC, CongestionWindowPushbackController already checks the
   "WebRTC-AddPacingToCongestionWindowPushback" field trial internally.
   If the trial is disabled (default), pacing_bytes_ is ignored during
   fill_ratio calculation.
3. Therefore, unconditionally populating msg.pacer_queue has no effect on
   GoogCC behavior when the trial is disabled, but allows other network
   controllers (e.g. SCReAM) to receive and inspect pacer queue metrics.

Bug: webrtc:447037083
Change-Id: I8f70181d1b263b56d09eb7143e494a582e5c831f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/490441
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48209}