Clean up WebRTC-Pacer-KeyframeFlushing trial.

This makes the "Enabled" path the new behavior, and removes all
references to the launched field trial.

Bug: webrtc:42221435
Change-Id: Icd32d0a5a464e63c30d43e35957add6b56f097d7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/489060
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Zoé Lepaul <zlep@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Zoé Lepaul <zlep@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48186}
diff --git a/experiments/field_trials.py b/experiments/field_trials.py
index a429fec..9ce3485 100755
--- a/experiments/field_trials.py
+++ b/experiments/field_trials.py
@@ -170,9 +170,6 @@
     FieldTrial('WebRTC-Pacer-FastRetransmissions',
                40235589,
                date(2024, 4, 1)),
-    FieldTrial('WebRTC-Pacer-KeyframeFlushing',
-               42221435,
-               date(2024, 4, 1)),
     FieldTrial('WebRTC-PayloadTypesInTransport',
                360058654,
                date(2025, 9, 11)),
diff --git a/modules/pacing/pacing_controller.cc b/modules/pacing/pacing_controller.cc
index 833879d..11c0af2 100644
--- a/modules/pacing/pacing_controller.cc
+++ b/modules/pacing/pacing_controller.cc
@@ -68,9 +68,6 @@
           field_trials.IsEnabled("WebRTC-Pacer-IgnoreTransportOverhead")),
       fast_retransmissions_(
           field_trials.IsEnabled("WebRTC-Pacer-FastRetransmissions")),
-      keyframe_flushing_(
-          configuration.keyframe_flushing ||
-          field_trials.IsEnabled("WebRTC-Pacer-KeyframeFlushing")),
       transport_overhead_per_packet_(DataSize::Zero()),
       send_burst_interval_(configuration.initial_pacer_config
                                ? configuration.initial_pacer_config->time_window
@@ -202,8 +199,7 @@
       << "SetPacingRate must be called before InsertPacket.";
   RTC_CHECK(packet->packet_type());
 
-  if (keyframe_flushing_ &&
-      packet->packet_type() == RtpPacketMediaType::kVideo &&
+  if (packet->packet_type() == RtpPacketMediaType::kVideo &&
       packet->is_key_frame() && packet->is_first_packet_of_frame() &&
       !packet_queue_.HasKeyframePackets(packet->Ssrc())) {
     // First packet of a keyframe (and no keyframe packets currently in the
diff --git a/modules/pacing/pacing_controller.h b/modules/pacing/pacing_controller.h
index 724ceae..b918d61 100644
--- a/modules/pacing/pacing_controller.h
+++ b/modules/pacing/pacing_controller.h
@@ -102,9 +102,8 @@
     // SetPacingRates() so that this limit will be upheld if
     // `drain_large_queues` is set.
     TimeDelta queue_time_limit = kMaxExpectedQueueLength;
-    // If the first packet of a keyframe is enqueued on a RTP stream, pacer
-    // skips forward to that packet and drops other enqueued packets on that
-    // stream, unless a keyframe is already being paced.
+    // TODO(bugs.webrtc.org/42221435): Remove when downstream usage is gone.
+    [[deprecated("Keyframe flushing is now always enabled.")]]
     bool keyframe_flushing = false;
     // Audio retransmission is prioritized before video retransmission packets.
     bool prioritize_audio_retransmission = false;
@@ -249,7 +248,6 @@
   const bool pace_audio_;
   const bool ignore_transport_overhead_;
   const bool fast_retransmissions_;
-  const bool keyframe_flushing_;
   DataSize transport_overhead_per_packet_;
   TimeDelta send_burst_interval_;
 
diff --git a/modules/pacing/pacing_controller_unittest.cc b/modules/pacing/pacing_controller_unittest.cc
index 0370248..416ccf1 100644
--- a/modules/pacing/pacing_controller_unittest.cc
+++ b/modules/pacing/pacing_controller_unittest.cc
@@ -2482,9 +2482,7 @@
   const uint32_t kSsrc = 12345;
   const uint32_t kRtxSsrc = 12346;
 
-  const FieldTrials trials =
-      CreateTestFieldTrials("WebRTC-Pacer-KeyframeFlushing/Enabled/");
-  auto pacer = std::make_unique<PacingController>(&clock_, &callback_, trials);
+  auto pacer = std::make_unique<PacingController>(&clock_, &callback_, trials_);
   EXPECT_CALL(callback_, GetRtxSsrcForMedia(kSsrc))
       .WillRepeatedly(Return(kRtxSsrc));
   pacer->SetPacerConfig(