Revert of Made MixerAudioSource a pure interface.

webrtc/modules/audio_mixer/audio_mixer_defines.h

Index: webrtc/modules/audio_mixer/audio_mixer_defines.h
diff --git a/webrtc/modules/audio_mixer/audio_mixer_defines.h b/webrtc/modules/audio_mixer/audio_mixer_defines.h
index 970a01de02e10663f656aad767fa0d1c74d91911..57e7f5027e4bf338843d8e1a3a0a0b48fbd7ef96 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_defines.h
+++ b/webrtc/modules/audio_mixer/audio_mixer_defines.h
@@ -35,8 +35,6 @@
     AudioFrameInfo audio_frame_info;
   };
 
-  virtual ~MixerAudioSource() = default;
-
   // The implementation of GetAudioFrameWithMuted should update
   // audio_frame with new audio every time it's called. Implementing
   // classes are allowed to return the same AudioFrame pointer on
@@ -45,6 +43,25 @@
   // mixer.
   virtual AudioFrameWithMuted GetAudioFrameWithMuted(int32_t id,
                                                      int sample_rate_hz) = 0;
+
+  // Returns true if the audio source was mixed this mix iteration.
+  bool IsMixed() const;
+
+  // Returns true if the audio source was mixed previous mix
+  // iteration.
+  bool WasMixed() const;
+
+  // Updates the mixed status.
+  int32_t SetIsMixed(bool mixed);
+
+  void ResetMixedStatus();
+
+ private:
+  bool is_mixed_;
+
+ protected:
+  MixerAudioSource();
+  virtual ~MixerAudioSource();
 };
 }  // namespace webrtc