Audio: Apply ABWENoTWCC trial to ReconfigureBitrateObserver

The Start() path short-circuits the BitrateAllocator registration gate
on `allocate_audio_without_feedback_`, but ReconfigureBitrateObserver()
does not. As a result, a Reconfigure() with an SDP that does not
negotiate transport-cc for audio unregisters the stream from the
BitrateAllocator, defeating the field trial.

Mirror the Start() condition so the trial behaves consistently across
the stream lifecycle.

Bug: None
Change-Id: I047bbf19827f907dd45c8fb3aefe61edab191b91
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/478860
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47910}

audio/audio_send_stream.cc

diff --git a/audio/audio_send_stream.cc b/audio/audio_send_stream.cc
index e9e72b4..11b62eb 100644
--- a/audio/audio_send_stream.cc
+++ b/audio/audio_send_stream.cc
@@ -787,7 +787,8 @@
 
   if (!new_config.has_dscp && new_config.min_bitrate_bps != -1 &&
       new_config.max_bitrate_bps != -1 &&
-      new_config.include_in_congestion_control_allocation) {
+      (allocate_audio_without_feedback_ ||
+       new_config.include_in_congestion_control_allocation)) {
     rtp_transport_->AccountForAudioPacketsInPacedSender(true);
     rtp_transport_->IncludeOverheadInPacedSender();
     // We may get a callback immediately as the observer is registered, so

audio/audio_send_stream_unittest.cc

diff --git a/audio/audio_send_stream_unittest.cc b/audio/audio_send_stream_unittest.cc
index 7857882..60f10ca 100644
--- a/audio/audio_send_stream_unittest.cc
+++ b/audio/audio_send_stream_unittest.cc
@@ -1074,5 +1074,41 @@
   send_stream->Stop();
 }
 
+// Verifies that ReconfigureBitrateObserver() preserves the
+// BitrateAllocator registration when the WebRTC-Audio-ABWENoTWCC field trial
+// is enabled and TWCC is not negotiated for audio (i.e.
+// `include_in_congestion_control_allocation` is false). The Start() path
+// already short-circuits on `allocate_audio_without_feedback_`; this test
+// ensures the Reconfigure() path applies the same short-circuit.
+TEST(AudioSendStreamTest, AbweNoTwccTrialKeepsBaRegistrationOnReconfigure) {
+  ConfigHelper helper(/*audio_bwe_enabled=*/false,
+                      /*expect_set_encoder_call=*/true,
+                      /*use_null_audio_processing=*/true);
+  helper.field_trials().Set("WebRTC-Audio-ABWENoTWCC", "Enabled");
+  auto send_stream = helper.CreateAudioSendStream();
+
+  // Start() registers the stream with the BitrateAllocator.
+  EXPECT_CALL(*helper.bitrate_allocator(), AddObserver(send_stream.get(), _))
+      .Times(1);
+  EXPECT_CALL(*helper.channel_send(), StartSend());
+  send_stream->Start();
+
+  // Reconfigure with a different bitrate range. The observer should be
+  // re-added (update path) and must not be removed.
+  auto new_config = helper.config();
+  new_config.min_bitrate_bps = 12000;
+  new_config.max_bitrate_bps = 80000;
+  EXPECT_CALL(*helper.bitrate_allocator(), AddObserver(send_stream.get(), _))
+      .Times(1);
+  EXPECT_CALL(*helper.bitrate_allocator(), RemoveObserver(send_stream.get()))
+      .Times(0);
+  send_stream->Reconfigure(new_config, nullptr);
+
+  EXPECT_CALL(*helper.bitrate_allocator(), RemoveObserver(send_stream.get()))
+      .Times(1);
+  EXPECT_CALL(*helper.channel_send(), StopSend());
+  send_stream->Stop();
+}
+
 }  // namespace test
 }  // namespace webrtc