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.h b/update_manager/variable.h
index ad2eefa..c47f488 100644
--- a/update_manager/variable.h
+++ b/update_manager/variable.h
@@ -166,7 +166,7 @@
 template<typename T>
 class Variable : public BaseVariable {
  public:
-  virtual ~Variable() {}
+  ~Variable() override {}
 
  protected:
   // Only allow to get values through the EvaluationContext class and not