Refactor the TrayAudio code so that it can be used by other platforms.

chromeos/audio/cras_audio_handler.cc

Index: chromeos/audio/cras_audio_handler.cc
diff --git a/chromeos/audio/cras_audio_handler.cc b/chromeos/audio/cras_audio_handler.cc
index 0045f84805023e6c473bb778a9acdf4b63d3b6b1..fd2d92e519457e8970001a1b9aca256f83d036cd 100644
--- a/chromeos/audio/cras_audio_handler.cc
+++ b/chromeos/audio/cras_audio_handler.cc
@@ -130,6 +130,10 @@ bool CrasAudioHandler::IsInputMutedForDevice(uint64 device_id) {
   return audio_pref_handler_->GetMuteValue(*device);
 }
 
+int CrasAudioHandler::GetOutputDefaultVolumeMuteThreshold() {
+  return kMuteThresholdPercent;
+}
+
 int CrasAudioHandler::GetOutputVolumePercent() {
   return output_volume_;
 }