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/prefs_interface.h b/prefs_interface.h
index 6c0a741..cdc6efe 100644
--- a/prefs_interface.h
+++ b/prefs_interface.h
@@ -18,6 +18,8 @@
 
 class PrefsInterface {
  public:
+  virtual ~PrefsInterface() {}
+
   // Gets a string |value| associated with |key|. Returns true on
   // success, false on failure (including when the |key| is not
   // present in the store).
@@ -52,8 +54,6 @@
   // Returns true if successfully deleted the file corresponding to
   // this key. Calling with non-existent keys does nothing.
   virtual bool Delete(const std::string& key) = 0;
-
-  virtual ~PrefsInterface() {}
 };
 
 }  // namespace chromeos_update_engine