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/fake_variable.h b/update_manager/fake_variable.h
index 9459fac..5b9facc 100644
--- a/update_manager/fake_variable.h
+++ b/update_manager/fake_variable.h
@@ -21,7 +21,7 @@
       : Variable<T>(name, mode) {}
   FakeVariable(const std::string& name, base::TimeDelta poll_interval)
       : Variable<T>(name, poll_interval) {}
-  virtual ~FakeVariable() {}
+  ~FakeVariable() override {}
 
   // Sets the next value of this variable to the passed |p_value| pointer. Once
   // returned by GetValue(), the pointer is released and has to be set again.