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_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 7fddfff..5fd353c 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -652,6 +652,8 @@
           media_stream_track.jitter_buffer_flushes);
       verifier.TestMemberIsNonNegative<uint64_t>(
           media_stream_track.delayed_packet_outage_samples);
+      verifier.TestMemberIsNonNegative<double>(
+          media_stream_track.relative_packet_arrival_delay);
     } else {
       verifier.TestMemberIsUndefined(media_stream_track.jitter_buffer_delay);
       verifier.TestMemberIsUndefined(
@@ -662,6 +664,8 @@
       verifier.TestMemberIsUndefined(media_stream_track.jitter_buffer_flushes);
       verifier.TestMemberIsUndefined(
           media_stream_track.delayed_packet_outage_samples);
+      verifier.TestMemberIsUndefined(
+          media_stream_track.relative_packet_arrival_delay);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }