Added RTCMediaStreamTrackStats.concealmentEvents

The number of concealment events. This counter increases every time a concealed sample is
synthesized after a non-concealed sample. That is, multiple consecutive concealed samples
will increase the concealedSamples count multiple times but is a single concealment event.

Bug: webrtc:8246
Change-Id: I7ef404edab765218b1f11e3128072c5391e83049
Reviewed-on: https://webrtc-review.googlesource.com/1221
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19881}
diff --git a/pc/rtcstatscollector_unittest.cc b/pc/rtcstatscollector_unittest.cc
index cb5a79f..14f669c 100644
--- a/pc/rtcstatscollector_unittest.cc
+++ b/pc/rtcstatscollector_unittest.cc
@@ -1555,6 +1555,7 @@
   voice_receiver_info.total_samples_received = 4567;
   voice_receiver_info.total_output_duration = 0.25;
   voice_receiver_info.concealed_samples = 123;
+  voice_receiver_info.concealment_events = 12;
 
   test_->CreateMockRtpSendersReceiversAndChannels(
       { std::make_pair(local_audio_track.get(), voice_sender_info_ssrc1),
@@ -1631,6 +1632,7 @@
   expected_remote_audio_track.total_samples_received = 4567;
   expected_remote_audio_track.total_samples_duration = 0.25;
   expected_remote_audio_track.concealed_samples = 123;
+  expected_remote_audio_track.concealment_events = 12;
   ASSERT_TRUE(report->Get(expected_remote_audio_track.id()));
   EXPECT_EQ(expected_remote_audio_track,
             report->Get(expected_remote_audio_track.id())->cast_to<