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}

call/rtp_transport_controller_send.cc

diff --git a/call/rtp_transport_controller_send.cc b/call/rtp_transport_controller_send.cc
index aa5b9c5..31c7451 100644
--- a/call/rtp_transport_controller_send.cc
+++ b/call/rtp_transport_controller_send.cc
@@ -117,8 +117,6 @@
       process_interval_(TimeDelta::PlusInfinity()),
       last_report_block_time_(env_.clock().CurrentTime()),
       initial_config_(env_),
-      add_pacing_to_cwin_(env_.field_trials().IsEnabled(
-          "WebRTC-AddPacingToCongestionWindowPushback")),
       reset_bwe_on_adapter_id_change_(
           env_.field_trials().IsEnabled("WebRTC-Bwe-ResetOnAdapterIdChange")),
       network_available_(false),
@@ -858,8 +856,7 @@
   RTC_DCHECK(controller_);
   ProcessInterval msg;
   msg.at_time = env_.clock().CurrentTime();
-  if (add_pacing_to_cwin_)
-    msg.pacer_queue = pacer_.QueueSizeData();
+  msg.pacer_queue = pacer_.QueueSizeData();
   PostUpdates(controller_->OnProcessInterval(msg));
 }
 

call/rtp_transport_controller_send.h

diff --git a/call/rtp_transport_controller_send.h b/call/rtp_transport_controller_send.h
index aaffe33..78f175f 100644
--- a/call/rtp_transport_controller_send.h
+++ b/call/rtp_transport_controller_send.h
@@ -229,7 +229,6 @@
   NetworkControllerConfig initial_config_ RTC_GUARDED_BY(worker_thread_);
   StreamsConfig streams_config_ RTC_GUARDED_BY(worker_thread_);
 
-  const bool add_pacing_to_cwin_;
   const bool reset_bwe_on_adapter_id_change_;
 
   DataSize transport_overhead_per_packet_ RTC_GUARDED_BY(worker_thread_) =