Remove GetVolume from media::AudioRendererSink.

media/audio/null_audio_sink.cc

Index: media/audio/null_audio_sink.cc
diff --git a/media/audio/null_audio_sink.cc b/media/audio/null_audio_sink.cc
index bf42d17abf9d422fe023a0a4d2a9efa868dd1524..6c164645918ae4eede6c305359773fc34998cc51 100644
--- a/media/audio/null_audio_sink.cc
+++ b/media/audio/null_audio_sink.cc
@@ -72,11 +72,6 @@ bool NullAudioSink::SetVolume(double volume) {
   return volume == 0.0;
 }
 
-void NullAudioSink::GetVolume(double* volume) {
-  // Audio is always muted.
-  *volume = 0.0;
-}
-
 void NullAudioSink::SetPlaying(bool is_playing) {
   base::AutoLock auto_lock(lock_);
   playing_ = is_playing;