Add support for deferred sequence numbering.

With this turned on, packets will be sequence number after the pacing
stage rather that during packetization.
This avoids a race where packets may be sent out of order, and paves
the way for the ability to cull packets from the pacer queue without
causing sequence number gaps.

For now, the feature is off by default. Follow-ups will enable it for
video and audio separately.

Bug: webrtc:11340, webrtc:12470
Change-Id: I6d411d8c85b9047e3e9b05ff4c2c3ed97c579aa1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208584
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34661}

modules/rtp_rtcp/source/rtp_rtcp_interface.h

@@ -146,6 +146,10 @@ class RtpRtcpInterface : public RtcpFeedbackSenderInterface {
     // https://tools.ietf.org/html/rfc3611#section-4.4 and #section-4.5
     bool non_sender_rtt_measurement = false;
 
+    // If true, sequence numbers are not assigned until after the pacer stage,
+    // in RtpSenderEgress.
+    bool use_deferred_sequencing = false;
+
    private:
     RTC_DISALLOW_COPY_AND_ASSIGN(Configuration);
   };