Expose relative packet arrival delay metric in stats API.

The metric is non-standard and documented in: https://github.com/henbos/webrtc-provisional-stats/pull/14

Bug: webrtc:10333
Change-Id: Ie5b4bbad5b1e2c9104742931529bab8f48f51f8c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125861
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26999}
diff --git a/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index f3a081b..a604663 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -460,6 +460,8 @@
       voice_receiver_info.jitter_buffer_flushes;
   audio_track_stats->delayed_packet_outage_samples =
       voice_receiver_info.delayed_packet_outage_samples;
+  audio_track_stats->relative_packet_arrival_delay =
+      voice_receiver_info.relative_packet_arrival_delay_seconds;
   return audio_track_stats;
 }