Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h

Index: webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h
diff --git a/webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h b/webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h
index 1b4a3c9da5b9361cc27b2bde49719c49dd75ebb7..d04181d55b11de7309d412dbd7dbf8d5f94a8521 100644
--- a/webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h
@@ -59,7 +59,7 @@ class MockDecoderDatabase : public DecoderDatabase {
   MOCK_METHOD1(SetActiveCngDecoder,
       int(uint8_t rtp_payload_type));
   MOCK_METHOD0(GetActiveCngDecoder,
-      AudioDecoder*());
+      ComfortNoiseDecoder*());
   MOCK_CONST_METHOD1(CheckPayloadTypes,
       int(const PacketList& packet_list));
 };