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..7ced9e29fa9e1c522c0c2de0cb3a1a9e990ec386 100644
--- a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
+++ b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
@@ -30,11 +30,11 @@ 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++) {
-    // TODO(andrew): this can easily overflow.
+    // TODO(aleloi): This can overflow. Convert to floats.
     energy += audio_frame.data_[position] * audio_frame.data_[position];
   }
   return energy;