Optionally allows TaskQueuePacedSender to coalesce send events.

With an optional parameter this allows the task-queue based paced
sender to mimic the old behavior and coalesce sending of packets in
order to reduce thread wakeups and provide opportunity for batching.
This is done by simply overriding the minimum time the thread should
sleep. The pacing controller will already handle the "late wakup" case
and send any packets as if it had been woken at the optimal time.

Bug: webrtc:10809
Change-Id: Iceea00693a4e87d39b0e0ee8bdabca081dff2cba
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175648
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31328}

modules/pacing/pacing_controller.h

@@ -146,6 +146,8 @@ class PacingController {
 
   bool Congested() const;
 
+  bool IsProbing() const;
+
  private:
   void EnqueuePacketInternal(std::unique_ptr<RtpPacketToSend> packet,
                              int priority);