GetTransportFeedbackVector() includes unreceived packets, sorted by seq-num

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
index f11949033af34c3e3eab6b23dde0805088e026d3..49bdf55479e13904c718110dae490572b2dbbe42 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
@@ -279,6 +279,7 @@ struct PacketFeedback {
         pacing_info(pacing_info) {}
 
   static constexpr int kNotAProbe = -1;
+  static constexpr int64_t kNotReceived = -1;
 
   // NOTE! The variable |creation_time_ms| is not used when testing equality.
   //       This is due to |creation_time_ms| only being used by SendTimeHistory
@@ -295,7 +296,8 @@ struct PacketFeedback {
   // Time corresponding to when this object was created.
   int64_t creation_time_ms;
   // Time corresponding to when the packet was received. Timestamped with the
-  // receiver's clock.
+  // receiver's clock. For unreceived packet, the sentinel value kNotReceived
+  // is used.
   int64_t arrival_time_ms;
   // Time corresponding to when the packet was sent, timestamped with the
   // sender's clock.