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/neteq/mock/mock_audio_decoder.h

@@ -31,6 +31,7 @@ class MockAudioDecoder : public AudioDecoder {
   MOCK_METHOD5(IncomingPacket, int(const uint8_t*, size_t, uint16_t, uint32_t,
                                    uint32_t));
   MOCK_METHOD0(ErrorCode, int());
+  MOCK_CONST_METHOD0(Channels, size_t());
   MOCK_CONST_METHOD0(codec_type, NetEqDecoder());
   MOCK_METHOD1(CodecSupported, bool(NetEqDecoder));
 };

webrtc/modules/audio_coding/neteq/mock/mock_external_decoder_pcm16b.h

@@ -41,6 +41,7 @@ class ExternalPcm16B : public AudioDecoder {
     *speech_type = ConvertSpeechType(1);
     return ret;
   }
+  size_t Channels() const override { return 1; }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalPcm16B);