Convert CNG into C++ and remove it from AudioDecoder

Broke out CNG from AudioDecoder as they didn't really share an interface.

Converted the CNG code to C++, to make initialization and resource handling easier. This includes several changes to the behavior, favoring RTC_CHECKs over returning error codes.

Review URL: https://codereview.webrtc.org/1868143002

Cr-Commit-Position: refs/heads/master@{#12491}

webrtc/modules/audio_coding/neteq/audio_decoder_impl.h

@@ -16,7 +16,6 @@
 #include "webrtc/engine_configurations.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/audio_coding/codecs/audio_decoder.h"
-#include "webrtc/modules/audio_coding/codecs/cng/webrtc_cng.h"
 #ifdef WEBRTC_CODEC_G722
 #include "webrtc/modules/audio_coding/codecs/g722/g722_interface.h"
 #endif
@@ -25,38 +24,6 @@
 
 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.