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

ash/system/tray/system_tray_notifier.cc

Index: ash/system/tray/system_tray_notifier.cc
diff --git a/ash/system/tray/system_tray_notifier.cc b/ash/system/tray/system_tray_notifier.cc
index 95b6789e87d75d0fe91ee6cddbf5e2ec33123a18..6a2cc79d79090b172bdb754ae2da70b210189207 100644
--- a/ash/system/tray/system_tray_notifier.cc
+++ b/ash/system/tray/system_tray_notifier.cc
@@ -29,6 +29,14 @@ void SystemTrayNotifier::RemoveAccessibilityObserver(
   accessibility_observers_.RemoveObserver(observer);
 }
 
+void SystemTrayNotifier::AddAudioObserver(AudioObserver* observer) {
+  audio_observers_.AddObserver(observer);
+}
+
+void SystemTrayNotifier::RemoveAudioObserver(AudioObserver* observer) {
+  audio_observers_.RemoveObserver(observer);
+}
+
 void SystemTrayNotifier::AddBluetoothObserver(BluetoothObserver* observer) {
   bluetooth_observers_.AddObserver(observer);
 }
@@ -170,6 +178,41 @@ void SystemTrayNotifier::NotifyAccessibilityModeChanged(
       OnAccessibilityModeChanged(notify));
 }
 
+void SystemTrayNotifier::NotifyAudioOutputVolumeChanged() {
+  FOR_EACH_OBSERVER(
+      AudioObserver,
+      audio_observers_,
+      OnOutputVolumeChanged());
+}
+
+void SystemTrayNotifier::NotifyAudioOutputMuteChanged() {
+  FOR_EACH_OBSERVER(
+      AudioObserver,
+      audio_observers_,
+      OnOutputMuteChanged());
+}
+
+void SystemTrayNotifier::NotifyAudioNodesChanged() {
+  FOR_EACH_OBSERVER(
+      AudioObserver,
+      audio_observers_,
+      OnAudioNodesChanged());
+}
+
+void SystemTrayNotifier::NotifyAudioActiveOutputNodeChanged() {
+  FOR_EACH_OBSERVER(
+      AudioObserver,
+      audio_observers_,
+      OnActiveOutputNodeChanged());
+}
+
+void SystemTrayNotifier::NotifyAudioActiveInputNodeChanged() {
+  FOR_EACH_OBSERVER(
+      AudioObserver,
+      audio_observers_,
+      OnActiveInputNodeChanged());
+}
+
 void SystemTrayNotifier::NotifyTracingModeChanged(bool value) {
   FOR_EACH_OBSERVER(
       TracingObserver,