Add estimatedPlayoutTimestamp to RTCInboundRTPStreamStats.

https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-estimatedplayouttimestamp

Partial implementation: currently only populated when a/v sync is enabled.

Bug: webrtc:7065
Change-Id: I8595cc848d080d7c3bef152462a9becf0e5a2196
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155621
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29581}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index f26c574..dd2eacd 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -446,6 +446,8 @@
   RTCStatsMember<double> total_decode_time;
   // https://henbos.github.io/webrtc-provisional-stats/#dom-rtcinboundrtpstreamstats-contenttype
   RTCStatsMember<std::string> content_type;
+  // TODO(asapersson): Currently only populated if audio/video sync is enabled.
+  RTCStatsMember<double> estimated_playout_timestamp;
   // TODO(hbos): This is only implemented for video; implement it for audio as
   // well.
   RTCStatsMember<std::string> decoder_implementation;