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/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.