update_engine: fix -Winconsistent-missing-override warning

BUG=chromium:453566
TEST=FEATURES="test" emerge-amd64-generic update_engine

Change-Id: Icf889d01021993e6b3175b3a15efe82069d2d2fc
Reviewed-on: https://chromium-review.googlesource.com/244411
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
diff --git a/update_manager/variable_unittest.cc b/update_manager/variable_unittest.cc
index 912d55c..642b5dd 100644
--- a/update_manager/variable_unittest.cc
+++ b/update_manager/variable_unittest.cc
@@ -28,8 +28,8 @@
   ~DefaultVariable() override {}
 
  protected:
-  virtual const T* GetValue(TimeDelta /* timeout */,
-                            string* /* errmsg */) {
+  const T* GetValue(TimeDelta /* timeout */,
+                    string* /* errmsg */) override {
     return new T();
   }