Made MixerAudioSource a pure interface.

webrtc/modules/audio_mixer/audio_mixer_defines.cc

 /*
  *  Copyright (c) 2016 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.
  */
 
 #include "webrtc/modules/audio_mixer/audio_mixer_defines.h"
 
 namespace webrtc {
 
 MixerAudioSource::MixerAudioSource() {}
 
 MixerAudioSource::~MixerAudioSource() {}
 
-bool MixerAudioSource::IsMixed() const {
-  return is_mixed_;
-}
-
-bool MixerAudioSource::WasMixed() const {
-  // Was mixed is the same as is mixed depending on perspective. This function
-  // is for the perspective of AudioMixerImpl.
-  return IsMixed();
-}
-
-int32_t MixerAudioSource::SetIsMixed(const bool mixed) {
-  is_mixed_ = mixed;
-  return 0;
-}
-
-void MixerAudioSource::ResetMixedStatus() {
-  is_mixed_ = false;
-}
 
 }  // namespace webrtc