Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/codecs/cng/cng.gypi

 # Copyright (c) 2011 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.
 
 {
   'targets': [
     {
       'target_name': 'cng',
       'type': 'static_library',
       'dependencies': [
         '<(webrtc_root)/common_audio/common_audio.gyp:common_audio',
         'audio_encoder_interface',
       ],
       'sources': [
         'audio_encoder_cng.cc',
         'audio_encoder_cng.h',
-        'cng_helpfuns.c',
-        'cng_helpfuns.h',
-        'webrtc_cng.c',
+        'webrtc_cng.cc',
         'webrtc_cng.h',
       ],
     },
   ], # targets
 }