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