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

ash/system/tray/system_tray_notifier.h

Index: ash/system/tray/system_tray_notifier.h
diff --git a/ash/system/tray/system_tray_notifier.h b/ash/system/tray/system_tray_notifier.h
index 6124040c8cfc49f679c9b8fa2dc83052300c0a3c..bfd7ff0aab4b6fd3c0694de35ff2faec0f41e424 100644
--- a/ash/system/tray/system_tray_notifier.h
+++ b/ash/system/tray/system_tray_notifier.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "ash/ash_export.h"
+#include "ash/system/audio/audio_observer.h"
 #include "ash/system/bluetooth/bluetooth_observer.h"
 #include "ash/system/chromeos/tray_tracing.h"
 #include "ash/system/date/clock_observer.h"
@@ -38,13 +39,16 @@ class NetworkStateNotifier;
 #endif
 
 class ASH_EXPORT SystemTrayNotifier {
-public:
+ public:
   SystemTrayNotifier();
   ~SystemTrayNotifier();
 
   void AddAccessibilityObserver(AccessibilityObserver* observer);
   void RemoveAccessibilityObserver(AccessibilityObserver* observer);
 
+  void AddAudioObserver(AudioObserver* observer);
+  void RemoveAudioObserver(AudioObserver* observer);
+
   void AddBluetoothObserver(BluetoothObserver* observer);
   void RemoveBluetoothObserver(BluetoothObserver* observer);
 
@@ -94,6 +98,11 @@ public:
 
   void NotifyAccessibilityModeChanged(
       AccessibilityNotificationVisibility notify);
+  void NotifyAudioOutputVolumeChanged();
+  void NotifyAudioOutputMuteChanged();
+  void NotifyAudioNodesChanged();
+  void NotifyAudioActiveOutputNodeChanged();
+  void NotifyAudioActiveInputNodeChanged();
   void NotifyTracingModeChanged(bool value);
   void NotifyRefreshBluetooth();
   void NotifyBluetoothDiscoveringChanged();
@@ -131,6 +140,7 @@ public:
 
  private:
   ObserverList<AccessibilityObserver> accessibility_observers_;
+  ObserverList<AudioObserver> audio_observers_;
   ObserverList<BluetoothObserver> bluetooth_observers_;
   ObserverList<CapsLockObserver> caps_lock_observers_;
   ObserverList<ClockObserver> clock_observers_;