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/fake_variable.h b/update_manager/fake_variable.h
index 5b9facc..9e51f7b 100644
--- a/update_manager/fake_variable.h
+++ b/update_manager/fake_variable.h
@@ -42,8 +42,8 @@
   // to the caller and the pointer is release from the FakeVariable. A second
   // call to GetValue() without reset() will return null and set the error
   // message.
-  virtual const T* GetValue(base::TimeDelta /* timeout */,
-                            std::string* errmsg) {
+  const T* GetValue(base::TimeDelta /* timeout */,
+                    std::string* errmsg) override {
     if (ptr_ == nullptr && errmsg != nullptr)
       *errmsg = this->GetName() + " is an empty FakeVariable";
     // Passes the pointer ownership to the caller.