Simple cleanups of AudioDecoder and AudioEncoder classes

webrtc/modules/audio_coding/codecs/pcm16b/include/audio_decoder_pcm16b.h

 /*
  *  Copyright (c) 2015 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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_CODING_CODECS_PCM16B_INCLUDE_AUDIO_DECODER_PCM16B_H_
 #define WEBRTC_MODULES_AUDIO_CODING_CODECS_PCM16B_INCLUDE_AUDIO_DECODER_PCM16B_H_
 
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/audio_coding/codecs/audio_decoder.h"
 
 namespace webrtc {
 
-class AudioDecoderPcm16B : public AudioDecoder {
+class AudioDecoderPcm16B final : public AudioDecoder {
  public:
-  AudioDecoderPcm16B();
+  explicit AudioDecoderPcm16B(size_t num_channels);
   void Reset() override;
   int PacketDuration(const uint8_t* encoded, size_t encoded_len) const override;
   size_t Channels() const override;
 
  protected:
   int DecodeInternal(const uint8_t* encoded,
                      size_t encoded_len,
                      int sample_rate_hz,
                      int16_t* decoded,
                      SpeechType* speech_type) override;
 
  private:
+  const size_t num_channels_;
   RTC_DISALLOW_COPY_AND_ASSIGN(AudioDecoderPcm16B);
 };
 
-class AudioDecoderPcm16BMultiCh : public AudioDecoderPcm16B {
- public:
-  explicit AudioDecoderPcm16BMultiCh(size_t num_channels);
-  size_t Channels() const override;
+}  // namespace webrtc
 
- private:
-  const size_t channels_;
-  RTC_DISALLOW_COPY_AND_ASSIGN(AudioDecoderPcm16BMultiCh);
-};
-
-}  // namespace webrtc
 #endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_PCM16B_INCLUDE_AUDIO_DECODER_PCM16B_H_