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.h b/prefs.h
index fd86dde..1c4ec4f 100644
--- a/prefs.h
+++ b/prefs.h
@@ -27,15 +27,15 @@
   bool Init(const base::FilePath& prefs_dir);
 
   // PrefsInterface methods.
-  bool GetString(const std::string& key, std::string* value);
-  bool SetString(const std::string& key, const std::string& value);
-  bool GetInt64(const std::string& key, int64_t* value);
-  bool SetInt64(const std::string& key, const int64_t value);
-  bool GetBoolean(const std::string& key, bool* value);
-  bool SetBoolean(const std::string& key, const bool value);
+  bool GetString(const std::string& key, std::string* value) override;
+  bool SetString(const std::string& key, const std::string& value) override;
+  bool GetInt64(const std::string& key, int64_t* value) override;
+  bool SetInt64(const std::string& key, const int64_t value) override;
+  bool GetBoolean(const std::string& key, bool* value) override;
+  bool SetBoolean(const std::string& key, const bool value) override;
 
-  bool Exists(const std::string& key);
-  bool Delete(const std::string& key);
+  bool Exists(const std::string& key) override;
+  bool Delete(const std::string& key) override;
 
  private:
   FRIEND_TEST(PrefsTest, GetFileNameForKey);