Moved MixerAudioSource and removed audio_mixer_defines.h.

webrtc/modules/audio_mixer/audio_frame_manipulator.h

Index: webrtc/modules/audio_mixer/audio_frame_manipulator.h
diff --git a/webrtc/modules/audio_mixer/audio_frame_manipulator.h b/webrtc/modules/audio_mixer/audio_frame_manipulator.h
index 4c8151d5463314549425cbee84806207332f2453..fa7d60b805ab584cb8599edcadc753e1f6aa7210 100644
--- a/webrtc/modules/audio_mixer/audio_frame_manipulator.h
+++ b/webrtc/modules/audio_mixer/audio_frame_manipulator.h
@@ -23,6 +23,9 @@ uint32_t AudioMixerCalculateEnergy(const AudioFrame& audio_frame);
 void NewMixerRampIn(AudioFrame* audio_frame);
 void NewMixerRampOut(AudioFrame* audio_frame);
 
+// Downmixes or upmixes a frame between stereo and mono.
+void RemixFrame(AudioFrame* frame, size_t target_number_of_channels);
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_MIXER_AUDIO_FRAME_MANIPULATOR_H_