Reland "Piping audio interruption metrics to API layer"

The metrics are now added as RTCNonStandardStatsMember objects in
RTCMediaStreamTrackStats. Unit tests are updated.

This is a reland of https://webrtc-review.googlesource.com/c/src/+/134303,
with fixes.

TBR=kwiberg@webrtc.org

Bug: webrtc:10549
Change-Id: I29dcc6fbfc69156715664e71acfa054c1b2d9038
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134500
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27806}

modules/audio_coding/include/audio_coding_module_typedefs.h

@@ -130,6 +130,10 @@ struct NetworkStatistics {
   uint64_t delayedPacketOutageSamples;
   // arrival delay of incoming packets
   uint64_t relativePacketArrivalDelayMs;
+  // number of audio interruptions
+  int32_t interruptionCount;
+  // total duration of audio interruptions
+  int32_t totalInterruptionDurationMs;
 };
 
 }  // namespace webrtc