Add the supporting code for the cast system tray integration

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 40e70501df9e1199992ac14135bc6bf0a21152dc..b02d6abb44043458b2db81e8c69ed0e14ec08376 100644
--- a/ash/system/tray/system_tray_notifier.cc
+++ b/ash/system/tray/system_tray_notifier.cc
@@ -38,6 +38,14 @@ void SystemTrayNotifier::RemoveBluetoothObserver(BluetoothObserver* observer) {
   bluetooth_observers_.RemoveObserver(observer);
 }
 
+void SystemTrayNotifier::AddCastObserver(CastObserver* observer) {
+  cast_observers_.AddObserver(observer);
+}
+
+void SystemTrayNotifier::RemoveCastObserver(CastObserver* observer) {
+  cast_observers_.RemoveObserver(observer);
+}
+
 void SystemTrayNotifier::AddClockObserver(ClockObserver* observer) {
   clock_observers_.AddObserver(observer);
 }