Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/comfort_noise.cc

Index: webrtc/modules/audio_coding/neteq/comfort_noise.cc
diff --git a/webrtc/modules/audio_coding/neteq/comfort_noise.cc b/webrtc/modules/audio_coding/neteq/comfort_noise.cc
index a5b08469beaec716e9c5e27424f3244a3ab6ab31..b957598924661410faa511d595a9c5f61e500295 100644
--- a/webrtc/modules/audio_coding/neteq/comfort_noise.cc
+++ b/webrtc/modules/audio_coding/neteq/comfort_noise.cc
@@ -29,15 +29,13 @@ void ComfortNoise::Reset() {
 int ComfortNoise::UpdateParameters(Packet* packet) {
   assert(packet);  // Existence is verified by caller.
   // Get comfort noise decoder.
-  AudioDecoder* cng_decoder = decoder_database_->GetDecoder(
-      packet->header.payloadType);
-  if (!cng_decoder) {
+  if (decoder_database_->SetActiveCngDecoder(packet->header.payloadType)
+      != kOK) {
     delete [] packet->payload;
     delete packet;
     return kUnknownPayloadType;
   }
-  decoder_database_->SetActiveCngDecoder(packet->header.payloadType);
-  CNG_dec_inst* cng_inst = cng_decoder->CngDecoderInstance();
+  CNG_dec_inst* cng_inst = decoder_database_->GetActiveCngDecoder();
   int16_t ret = WebRtcCng_UpdateSid(cng_inst,
                                     packet->payload,
                                     packet->payload_length);
@@ -71,12 +69,11 @@ int ComfortNoise::Generate(size_t requested_length,
   }
   output->AssertSize(number_of_samples);
   // Get the decoder from the database.
-  AudioDecoder* cng_decoder = decoder_database_->GetActiveCngDecoder();
-  if (!cng_decoder) {
+  CNG_dec_inst* cng_inst = decoder_database_->GetActiveCngDecoder();
+  if (!cng_inst) {
     LOG(LS_ERROR) << "Unknwown payload type";
     return kUnknownPayloadType;
   }
-  CNG_dec_inst* cng_inst = cng_decoder->CngDecoderInstance();
   // The expression &(*output)[0][0] is a pointer to the first element in
   // the first channel.
   if (WebRtcCng_Generate(cng_inst, &(*output)[0][0], number_of_samples,