Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc

Index: webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc
diff --git a/webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc b/webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc
index 929f844f63b29e177b613a44a132de2117fa365a..0da4d640f3d905dd514574785c32f687ac531e60 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc
+++ b/webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc
@@ -231,8 +231,8 @@ TEST(DecoderDatabase, IF_ISAC(ActiveDecoders)) {
 
   // Set active CNG codec.
   EXPECT_EQ(DecoderDatabase::kOK, db.SetActiveCngDecoder(13));
-  decoder = db.GetActiveCngDecoder();
-  ASSERT_FALSE(decoder == NULL);  // Should get a decoder here.
+  ComfortNoiseDecoder* cng = db.GetActiveCngDecoder();
+  ASSERT_FALSE(cng == NULL);  // Should get a decoder here.
 
   // Remove the active CNG decoder, and verify that the active becomes NULL.
   EXPECT_EQ(DecoderDatabase::kOK, db.Remove(13));