Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h

Index: webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h
diff --git a/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h b/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h
index b52665d9c0a6b026ff27c5dd7708535d1b4c28dd..246e9519da486a1e7a35d26d9d59042cb1461229 100644
--- a/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h
+++ b/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.h
@@ -21,11 +21,6 @@
 
 namespace webrtc {
 
-// Deleter for use with unique_ptr.
-struct CngInstDeleter {
-  void operator()(CNG_enc_inst* ptr) const { WebRtcCng_FreeEnc(ptr); }
-};
-
 class Vad;
 
 class AudioEncoderCng final : public AudioEncoder {
@@ -84,7 +79,7 @@ class AudioEncoderCng final : public AudioEncoder {
   std::vector<uint32_t> rtp_timestamps_;
   bool last_frame_active_;
   std::unique_ptr<Vad> vad_;
-  std::unique_ptr<CNG_enc_inst, CngInstDeleter> cng_inst_;
+  std::unique_ptr<ComfortNoiseEncoder> cng_encoder_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(AudioEncoderCng);
 };