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/media/base/media_channel.h b/media/base/media_channel.h
index 4106949..bd703dc 100644
--- a/media/base/media_channel.h
+++ b/media/base/media_channel.h
@@ -487,6 +487,8 @@
   uint64_t jitter_buffer_flushes = 0;
   // Number of samples expanded due to delayed packets.
   uint64_t delayed_packet_outage_samples = 0;
+  // Arrival delay of received audio packets.
+  double relative_packet_arrival_delay_seconds = 0.0;
 };
 
 struct VideoSenderInfo : public MediaSenderInfo {