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}

modules/audio_coding/include/audio_coding_module_typedefs.h

@@ -117,6 +117,8 @@ struct NetworkStatistics {
   int maxWaitingTimeMs;
   // added samples in off mode due to packet loss
   size_t addedSamples;
+  // count of the number of buffer flushes
+  uint64_t packetBufferFlushes;
 };
 
 }  // namespace webrtc