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/random_provider.h b/update_manager/random_provider.h
index fef5c35..7c22863 100644
--- a/update_manager/random_provider.h
+++ b/update_manager/random_provider.h
@@ -13,7 +13,7 @@
 // Provider of random values.
 class RandomProvider : public Provider {
  public:
-  virtual ~RandomProvider() {}
+  ~RandomProvider() override {}
 
   // Return a random number every time it is requested. Note that values
   // returned by the variables are cached by the EvaluationContext, so the