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

ash/test/test_system_tray_delegate.h

Index: ash/test/test_system_tray_delegate.h
diff --git a/ash/test/test_system_tray_delegate.h b/ash/test/test_system_tray_delegate.h
index 244eac211fe3ea23d2335a2baf00a8e03bfd7fb8..e25c8c83869b9c52853da583f4b97149bd49d0de 100644
--- a/ash/test/test_system_tray_delegate.h
+++ b/ash/test/test_system_tray_delegate.h
@@ -45,6 +45,7 @@ class TestSystemTrayDelegate : public DefaultSystemTrayDelegate {
   // Overridden from SystemTrayDelegate:
   LoginStatus GetUserLoginStatus() const override;
   bool IsUserSupervised() const override;
+  void GetSystemUpdateInfo(UpdateInfo* info) const override;
   bool ShouldShowDisplayNotification() override;
   bool GetSessionStartTime(base::TimeTicks* session_start_time) override;
   bool GetSessionLengthLimit(base::TimeDelta* session_length_limit) override;