Add the supporting code for the cast system tray integration

ash/system/tray/system_tray_delegate.cc

Index: ash/system/tray/system_tray_delegate.cc
diff --git a/ash/system/tray/system_tray_delegate.cc b/ash/system/tray/system_tray_delegate.cc
index eaab406a483f6dd1bd482ecc764110b93481efb7..ed999b6a970e8b07a37f975697aa56482d33cd55 100644
--- a/ash/system/tray/system_tray_delegate.cc
+++ b/ash/system/tray/system_tray_delegate.cc
@@ -223,6 +223,10 @@ bool SystemTrayDelegate::GetBluetoothDiscovering() {
 void SystemTrayDelegate::ChangeProxySettings() {
 }
 
+CastConfigDelegate* SystemTrayDelegate::GetCastConfigDelegate() const {
+  return nullptr;
+}
+
 NetworkingConfigDelegate* SystemTrayDelegate::GetNetworkingConfigDelegate()
     const {
   return nullptr;