Add new decoding statistics for muted output

This change adds a new statistic for logging how many calls to
NetEq::GetAudio resulted in a "muted output". A muted output happens
if the packet stream has been dead for some time (and the last decoded
packet was not comfort noise).

BUG=webrtc:5606
BUG=b/31256483

Review-Url: https://codereview.webrtc.org/2341293002
Cr-Commit-Position: refs/heads/master@{#14302}
diff --git a/webrtc/modules/audio_coding/acm2/call_statistics.h b/webrtc/modules/audio_coding/acm2/call_statistics.h
index 888afea..3993319 100644
--- a/webrtc/modules/audio_coding/acm2/call_statistics.h
+++ b/webrtc/modules/audio_coding/acm2/call_statistics.h
@@ -37,8 +37,9 @@
   ~CallStatistics() {}
 
   // Call this method to indicate that NetEq engaged in decoding. |speech_type|
-  // is the audio-type according to NetEq.
-  void DecodedByNetEq(AudioFrame::SpeechType speech_type);
+  // is the audio-type according to NetEq, and |muted| indicates if the decoded
+  // frame was produced in muted state.
+  void DecodedByNetEq(AudioFrame::SpeechType speech_type, bool muted);
 
   // Call this method to indicate that a decoding call resulted in generating
   // silence, i.e. call to NetEq is bypassed and the output audio is zero.