Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/comfort_noise.h

Index: webrtc/modules/audio_coding/neteq/comfort_noise.h
diff --git a/webrtc/modules/audio_coding/neteq/comfort_noise.h b/webrtc/modules/audio_coding/neteq/comfort_noise.h
index 1fc225866370c0d254e285fec07b0e3b02db9fa5..f877bf63efb05e4bc32ba9cca637ce645abb8a8c 100644
--- a/webrtc/modules/audio_coding/neteq/comfort_noise.h
+++ b/webrtc/modules/audio_coding/neteq/comfort_noise.h
@@ -38,8 +38,7 @@ class ComfortNoise {
         first_call_(true),
         overlap_length_(5 * fs_hz_ / 8000),
         decoder_database_(decoder_database),
-        sync_buffer_(sync_buffer),
-        internal_error_code_(0) {
+        sync_buffer_(sync_buffer) {
   }
 
   // Resets the state. Should be called before each new comfort noise period.