Revert of Made MixerAudioSource a pure interface.

webrtc/modules/audio_mixer/audio_mixer_defines.h

 /*
  *  Copyright (c) 2011 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.
  */
 
@@ skipping 17 matching lines @@
               // implicitly interpreted as zero. Other fields in audio_frame
               // may be read and should contain meaningful values.
     kError    // audio_frame will not be used.
   };
 
   struct AudioFrameWithMuted {
     AudioFrame* audio_frame;
     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
   // different calls. The pointer must stay valid until the next
   // mixing call or until this audio source is disconnected from the
   // 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
 
 #endif  // WEBRTC_MODULES_AUDIO_MIXER_AUDIO_MIXER_DEFINES_H_