Move AudioDecoder and related stuff to the api/ directory

webrtc/modules/audio_coding/codecs/builtin_audio_decoder_factory.h

 /*
  *  Copyright (c) 2016 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.
  */
 
+// This file is for backwards compatibility only! Use
+// webrtc/api/audio_codecs/builtin_audio_decoder_factory.h instead!
+// TODO(kwiberg): Remove it.
+
 #ifndef WEBRTC_MODULES_AUDIO_CODING_CODECS_BUILTIN_AUDIO_DECODER_FACTORY_H_
 #define WEBRTC_MODULES_AUDIO_CODING_CODECS_BUILTIN_AUDIO_DECODER_FACTORY_H_
 
-#include <memory>
-
-#include "webrtc/base/scoped_ref_ptr.h"
-#include "webrtc/modules/audio_coding/codecs/audio_decoder_factory.h"
-
-namespace webrtc {
-
-// Creates a new factory that can create the built-in types of audio decoders.
-// NOTE: This function is still under development and may change without notice.
-rtc::scoped_refptr<AudioDecoderFactory> CreateBuiltinAudioDecoderFactory();
-
-}  // namespace webrtc
+#include "webrtc/api/audio_codecs/builtin_audio_decoder_factory.h"
 
 #endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_BUILTIN_AUDIO_DECODER_FACTORY_H_