Made MixerAudioSource a pure interface.

webrtc/modules/audio_mixer/audio_frame_manipulator.cc

Index: webrtc/modules/audio_mixer/audio_frame_manipulator.cc
diff --git a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
index aafeedaeda9748100fe98b49dba72c516234ba19..e492e31e07f0dfb6cac76282f7d3c67caca22629 100644
--- a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
+++ b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
@@ -30,7 +30,7 @@ const float kRampArray[] = {
 const size_t kRampSize = sizeof(kRampArray) / sizeof(kRampArray[0]);
 }  // namespace
 
-uint32_t NewMixerCalculateEnergy(const AudioFrame& audio_frame) {
+uint32_t AudioMixerCalculateEnergy(const AudioFrame& audio_frame) {
   uint32_t energy = 0;
   for (size_t position = 0; position < audio_frame.samples_per_channel_;
        position++) {