Make AudioDecoder stateless

The channels_ member varable is removed from the base class, and the
associated accessor function is changed to Channels() which is a pure
virtual function.

R=jmarusic@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43779004

Cr-Commit-Position: refs/heads/master@{#8775}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8775 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/audio_decoder.h

@@ -31,8 +31,8 @@ class AudioDecoder {
   // Used by PacketDuration below. Save the value -1 for errors.
   enum { kNotImplemented = -2 };
 
-  AudioDecoder() : channels_(1) {}
-  virtual ~AudioDecoder() {}
+  AudioDecoder() = default;
+  virtual ~AudioDecoder() = default;
 
   // Decodes |encode_len| bytes from |encoded| and writes the result in
   // |decoded|. The maximum bytes allowed to be written into |decoded| is
@@ -97,7 +97,7 @@ class AudioDecoder {
   // isn't a CNG decoder, don't call this method.
   virtual CNG_dec_inst* CngDecoderInstance();
 
-  size_t channels() const { return channels_; }
+  virtual size_t Channels() const = 0;
 
  protected:
   static SpeechType ConvertSpeechType(int16_t type);
@@ -114,8 +114,6 @@ class AudioDecoder {
                                       int16_t* decoded,
                                       SpeechType* speech_type);
 
-  size_t channels_;
-
  private:
   DISALLOW_COPY_AND_ASSIGN(AudioDecoder);
 };

webrtc/modules/audio_coding/codecs/isac/audio_encoder_isac_t.h

@@ -81,6 +81,7 @@ class AudioEncoderDecoderIsacT : public AudioEncoder, public AudioDecoder {
                      uint32_t rtp_timestamp,
                      uint32_t arrival_timestamp) override;
   int ErrorCode() override;
+  size_t Channels() const override { return 1; }
 
  protected:
   // AudioEncoder protected method.