ash: Add a unit test for the system update tray item

ash/test/test_system_tray_delegate.cc

Index: ash/test/test_system_tray_delegate.cc
diff --git a/ash/test/test_system_tray_delegate.cc b/ash/test/test_system_tray_delegate.cc
index 0d5ef0e39fd143451eacec5bd503550d54729d02..a14e174d24e359a25f0e4b77780674c464ce99a1 100644
--- a/ash/test/test_system_tray_delegate.cc
+++ b/ash/test/test_system_tray_delegate.cc
@@ -73,6 +73,14 @@ bool TestSystemTrayDelegate::IsUserSupervised() const {
   return login_status_ == LoginStatus::SUPERVISED;
 }
 
+void TestSystemTrayDelegate::GetSystemUpdateInfo(UpdateInfo* info) const {
+  DCHECK(info);
+  // In tests, default to not having an update required.
+  info->severity = UpdateInfo::UPDATE_NORMAL;
+  info->update_required = false;
+  info->factory_reset_required = false;
+}
+
 bool TestSystemTrayDelegate::ShouldShowDisplayNotification() {
   return should_show_display_notification_;
 }