Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
 import("../../build/webrtc.gni")
@@ skipping 129 matching lines @@
   include_dirs = [
     "../../..",
     "codecs/cng/include",
   ]
 }
 
 source_set("cng") {
   sources = [
     "codecs/cng/audio_encoder_cng.cc",
     "codecs/cng/audio_encoder_cng.h",
-    "codecs/cng/cng_helpfuns.c",
-    "codecs/cng/cng_helpfuns.h",
-    "codecs/cng/webrtc_cng.c",
+    "codecs/cng/webrtc_cng.cc",
     "codecs/cng/webrtc_cng.h",
   ]
 
   configs += [ "../..:common_config" ]
 
   public_configs = [
     "../..:common_inherited_config",
     ":cng_config",
   ]
 
@@ skipping 682 matching lines @@
       defines += [ "WEBRTC_CODEC_ISACFX" ]
       deps += [ ":isac_fix" ]
     } else {
       defines += [ "WEBRTC_CODEC_ISAC" ]
       deps += [ ":isac" ]
     }
     defines += [ "WEBRTC_CODEC_G722" ]
     deps += [ ":g722" ]
   }
 }