Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/audio_decoder_impl.h

Index: webrtc/modules/audio_coding/neteq/audio_decoder_impl.h
diff --git a/webrtc/modules/audio_coding/neteq/audio_decoder_impl.h b/webrtc/modules/audio_coding/neteq/audio_decoder_impl.h
index bc8bdd9626d19f04d836310386af2f232fc8c5b6..049ddf4a552539893ad5af4fbff900a40e4c0557 100644
--- a/webrtc/modules/audio_coding/neteq/audio_decoder_impl.h
+++ b/webrtc/modules/audio_coding/neteq/audio_decoder_impl.h
@@ -25,38 +25,6 @@
 
 namespace webrtc {
 
-// AudioDecoderCng is a special type of AudioDecoder. It inherits from
-// AudioDecoder just to fit in the DecoderDatabase. None of the class methods
-// should be used, except constructor, destructor, and accessors.
-// TODO(hlundin): Consider the possibility to create a super-class to
-// AudioDecoder that is stored in DecoderDatabase. Then AudioDecoder and a
-// specific CngDecoder class could both inherit from that class.
-class AudioDecoderCng : public AudioDecoder {
- public:
-  explicit AudioDecoderCng();
-  ~AudioDecoderCng() override;
-  void Reset() override;
-  int IncomingPacket(const uint8_t* payload,
-                     size_t payload_len,
-                     uint16_t rtp_sequence_number,
-                     uint32_t rtp_timestamp,
-                     uint32_t arrival_timestamp) override;
-
-  CNG_dec_inst* CngDecoderInstance() override;
-  size_t Channels() const override;
-
- protected:
-  int DecodeInternal(const uint8_t* encoded,
-                     size_t encoded_len,
-                     int sample_rate_hz,
-                     int16_t* decoded,
-                     SpeechType* speech_type) override;
-
- private:
-  CNG_dec_inst* dec_state_;
-  RTC_DISALLOW_COPY_AND_ASSIGN(AudioDecoderCng);
-};
-
 using NetEqDecoder = acm2::RentACodec::NetEqDecoder;
 
 // Returns true if |codec_type| is supported.