Expose jitter buffer flushes metric in new getStats api.

Origin trial experiment proposal (new statistic part):
https://docs.google.com/document/d/1stYIZhEmDZ7NJF9gjjsM66eLFJUdc-14a3QutrFbIwI/edit?ts=5bf5535c#

Bug: chromium:907113
Change-Id: I1d005291f9b47665f70c26148dbdcbb55564bef8
Reviewed-on: https://webrtc-review.googlesource.com/c/111505
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Ruslan Burakov <kuddai@google.com>
Cr-Commit-Position: refs/heads/master@{#25768}
diff --git a/call/audio_receive_stream.h b/call/audio_receive_stream.h
index f792aa2..fe609de 100644
--- a/call/audio_receive_stream.h
+++ b/call/audio_receive_stream.h
@@ -71,6 +71,7 @@
     int32_t decoding_plc_cng = 0;
     int32_t decoding_muted_output = 0;
     int64_t capture_start_ntp_time_ms = 0;
+    uint64_t jitter_buffer_flushes = 0;
   };
 
   struct Config {