Reland of StartTimestamp generated randomly in RtpSender constructor

webrtc/modules/rtp_rtcp/source/rtp_sender.h

Index: webrtc/modules/rtp_rtcp/source/rtp_sender.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender.h b/webrtc/modules/rtp_rtcp/source/rtp_sender.h
index dac96edd263493a88dc966bf122c8f52c7421499..4331e2659e81a4d920f587585ce46fd3dcfedb5b 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender.h
@@ -90,8 +90,8 @@
   void GetDataCounters(StreamDataCounters* rtp_stats,
                        StreamDataCounters* rtx_stats) const;
 
-  uint32_t StartTimestamp() const;
-  void SetStartTimestamp(uint32_t timestamp, bool force);
+  uint32_t TimestampOffset() const;
+  void SetTimestampOffset(uint32_t timestamp);
 
   uint32_t GenerateNewSSRC();
   void SetSSRC(uint32_t ssrc);
@@ -408,8 +408,7 @@
   BitrateStatisticsObserver* const bitrate_callback_;
 
   // RTP variables
-  bool start_timestamp_forced_ GUARDED_BY(send_critsect_);
-  uint32_t start_timestamp_ GUARDED_BY(send_critsect_);
+  uint32_t timestamp_offset_ GUARDED_BY(send_critsect_);
   SSRCDatabase* const ssrc_db_;
   uint32_t remote_ssrc_ GUARDED_BY(send_critsect_);
   bool sequence_number_forced_ GUARDED_BY(send_critsect_);