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/pc/rtcstats_integrationtest.cc b/pc/rtcstats_integrationtest.cc
index f9375f7..46687ba 100644
--- a/pc/rtcstats_integrationtest.cc
+++ b/pc/rtcstats_integrationtest.cc
@@ -604,11 +604,14 @@
           media_stream_track.concealed_samples);
       verifier.TestMemberIsNonNegative<uint64_t>(
           media_stream_track.concealment_events);
+      verifier.TestMemberIsNonNegative<uint64_t>(
+          media_stream_track.jitter_buffer_flushes);
     } else {
       verifier.TestMemberIsUndefined(media_stream_track.jitter_buffer_delay);
       verifier.TestMemberIsUndefined(media_stream_track.total_samples_received);
       verifier.TestMemberIsUndefined(media_stream_track.concealed_samples);
       verifier.TestMemberIsUndefined(media_stream_track.concealment_events);
+      verifier.TestMemberIsUndefined(media_stream_track.jitter_buffer_flushes);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }