update_engine: Add override when possible.

Google Style Guide requires to include the "override" keyword
when overriding a method on a derived class, so the compiler will
catch errors if the method is not overriding a member of the base
class.

This patch introduces the "override" keyword when possible.

BUG=None
TEST=FEATURES=test emerge-link update_engine

Change-Id: Ie83d115c5730f3b35b3d95859a54bc1a48e0be7b
Reviewed-on: https://chromium-review.googlesource.com/228928
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/update_manager/variable_unittest.cc b/update_manager/variable_unittest.cc
index 7944c38..912d55c 100644
--- a/update_manager/variable_unittest.cc
+++ b/update_manager/variable_unittest.cc
@@ -25,7 +25,7 @@
       : Variable<T>(name, mode) {}
   DefaultVariable(const string& name, const TimeDelta& poll_interval)
       : Variable<T>(name, poll_interval) {}
-  virtual ~DefaultVariable() {}
+  ~DefaultVariable() override {}
 
  protected:
   virtual const T* GetValue(TimeDelta /* timeout */,