Add ability to control TaskQueuePacedSender holdback window.

Holdback window can be specified as absolute time and in terms of packet
send times. Example:
WebRTC-TaskQueuePacer/Enabled,holdback_window:20ms,holdback_packet:3/

If current conditions have us running with 2000kbps pacing rate and
1250byte (10kbit) packets, each packet send time is 5ms.
The holdback window would then be min(20ms, 3*5ms) = 15ms.

The default is like before 1ms and packets no take into account when
TQ pacer is used, parameters have no effect with legacy process thread
pacer.

Bug: webrtc:10809
Change-Id: I800de05107e2d4df461eabaaf1ca04fb4c5de51e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/233421
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35266}
diff --git a/call/rtp_transport_controller_send.h b/call/rtp_transport_controller_send.h
index f1b90c7..ac4213d 100644
--- a/call/rtp_transport_controller_send.h
+++ b/call/rtp_transport_controller_send.h
@@ -128,6 +128,16 @@
   void OnRemoteNetworkEstimate(NetworkStateEstimate estimate) override;
 
  private:
+  struct PacerSettings {
+    explicit PacerSettings(const WebRtcKeyValueConfig* trials);
+
+    bool use_task_queue_pacer() const { return !tq_disabled.Get(); }
+
+    FieldTrialFlag tq_disabled;  // Kill-switch not normally used.
+    FieldTrialParameter<TimeDelta> holdback_window;
+    FieldTrialParameter<int> holdback_packets;
+  };
+
   void MaybeCreateControllers() RTC_RUN_ON(task_queue_);
   void UpdateInitialConstraints(TargetRateConstraints new_contraints)
       RTC_RUN_ON(task_queue_);
@@ -158,7 +168,7 @@
   std::map<std::string, rtc::NetworkRoute> network_routes_;
   bool pacer_started_;
   const std::unique_ptr<ProcessThread> process_thread_;
-  const bool use_task_queue_pacer_;
+  const PacerSettings pacer_settings_;
   std::unique_ptr<PacedSender> process_thread_pacer_;
   std::unique_ptr<TaskQueuePacedSender> task_queue_pacer_;