Don't write pacer exit timestamp without pacer

And allow populating network2 timestamp if we want to preserve pacer
timestamp.

Bug: webrtc:8853
Change-Id: I895d5ce8a9cca8ceeec3bf08e2eff02bf3b2f5fd
Reviewed-on: https://webrtc-review.googlesource.com/48640
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21937}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -96,6 +96,9 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
     RtpKeepAliveConfig keepalive_config;
     RtcpIntervalConfig rtcp_interval_config;
 
+    // Update network2 instead of pacer_exit field of video timing extension.
+    bool populate_network2_timestamp = false;
+
    private:
     RTC_DISALLOW_COPY_AND_ASSIGN(Configuration);
   };