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/call/audio_receive_stream.h b/call/audio_receive_stream.h
index 1f8ad10..55c1af7 100644
--- a/call/audio_receive_stream.h
+++ b/call/audio_receive_stream.h
@@ -87,6 +87,8 @@
     double relative_packet_arrival_delay_seconds = 0.0;
     int32_t interruption_count = 0;
     int32_t total_interruption_duration_ms = 0;
+    // https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-estimatedplayouttimestamp
+    absl::optional<int64_t> estimated_playout_ntp_timestamp_ms;
   };
 
   struct Config {