Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/decoder_database.h

Index: webrtc/modules/audio_coding/neteq/decoder_database.h
diff --git a/webrtc/modules/audio_coding/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 01ff0c9fdb365fcccc24153352d352fbcc79ff2f..343e14fcbcddea82f9f687be5213a42b6f23eaa9 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -142,7 +142,7 @@ class DecoderDatabase {
 
   // Returns the current active comfort noise decoder, or NULL if no active
   // comfort noise decoder exists.
-  virtual AudioDecoder* GetActiveCngDecoder();
+  virtual CNG_dec_inst* GetActiveCngDecoder();
 
   // Returns kOK if all packets in |packet_list| carry payload types that are
   // registered in the database. Otherwise, returns kDecoderNotFound.
@@ -154,6 +154,7 @@ class DecoderDatabase {
   DecoderMap decoders_;
   int active_decoder_;
   int active_cng_decoder_;
+  CNG_dec_inst* active_cng_dec_inst_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(DecoderDatabase);
 };