stats: implement candidate-pair lastPacket(Sent|Received)Timestamp

https://w3c.github.io/webrtc-stats/#dom-rtcicecandidatepairstats-lastpacketsenttimestamp
https://w3c.github.io/webrtc-stats/#dom-rtcicecandidatepairstats-lastpacketreceivedtimestamp

which are useful together with the ice-restart-necessary logic mentioned
in
  https://w3c.github.io/webrtc-pc/#dictionary-rtcofferoptions-members

BUG=webrtc:14619

Change-Id: I4a8ab00a37fbd4af8b948720c83787cbdfc6b9a3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/281281
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#38534}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index f103b1d..b62b0aa 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -222,6 +222,8 @@
   RTCStatsMember<uint64_t> consent_requests_sent;
   RTCStatsMember<uint64_t> packets_discarded_on_send;
   RTCStatsMember<uint64_t> bytes_discarded_on_send;
+  RTCStatsMember<double> last_packet_received_timestamp;
+  RTCStatsMember<double> last_packet_sent_timestamp;
 };
 
 // https://w3c.github.io/webrtc-stats/#icecandidate-dict*