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/api/statscollector.cc b/webrtc/api/statscollector.cc
index d21fc30..4dd5305 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -143,6 +143,8 @@
     { StatsReport::kStatsValueNameDecodingCTN, info.decoding_calls_to_neteq },
     { StatsReport::kStatsValueNameDecodingCTSG,
       info.decoding_calls_to_silence_generator },
+    { StatsReport::kStatsValueNameDecodingMutedOutput,
+      info.decoding_muted_output },
     { StatsReport::kStatsValueNameDecodingNormal, info.decoding_normal },
     { StatsReport::kStatsValueNameDecodingPLC, info.decoding_plc },
     { StatsReport::kStatsValueNameDecodingPLCCNG, info.decoding_plc_cng },