Add the supporting code for the cast system tray integration

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 29bba2aa54d227fce4bd94163035a13b5f40dd6a..4ea4cb41ee39be349b15e7144c7e6aa97aadb5bf 100644
--- a/ash/system/tray/system_tray_notifier.h
+++ b/ash/system/tray/system_tray_notifier.h
@@ -11,6 +11,7 @@
 #include "ash/ash_export.h"
 #include "ash/system/audio/audio_observer.h"
 #include "ash/system/bluetooth/bluetooth_observer.h"
+#include "ash/system/cast/cast_observer.h"
 #include "ash/system/chromeos/tray_tracing.h"
 #include "ash/system/date/clock_observer.h"
 #include "ash/system/ime/ime_observer.h"
@@ -56,6 +57,9 @@ class ASH_EXPORT SystemTrayNotifier {
   void AddBluetoothObserver(BluetoothObserver* observer);
   void RemoveBluetoothObserver(BluetoothObserver* observer);
 
+  void AddCastObserver(CastObserver* observer);
+  void RemoveCastObserver(CastObserver* observer);
+
   void AddClockObserver(ClockObserver* observer);
   void RemoveClockObserver(ClockObserver* observer);
 
@@ -153,6 +157,7 @@ class ASH_EXPORT SystemTrayNotifier {
   ObserverList<AccessibilityObserver> accessibility_observers_;
   ObserverList<AudioObserver> audio_observers_;
   ObserverList<BluetoothObserver> bluetooth_observers_;
+  ObserverList<CastObserver> cast_observers_;
   ObserverList<ClockObserver> clock_observers_;
   ObserverList<IMEObserver> ime_observers_;
   ObserverList<LocaleObserver> locale_observers_;