Standardize usage of virtual/override/final in tools/gn/

tools/gn/config.h

Index: tools/gn/config.h
diff --git a/tools/gn/config.h b/tools/gn/config.h
index f08b85915302f6f22cc8848c5eaaa6e96bbb2934..9c7571d5b499f753f298d1adfdc431a53dc3c6ed 100644
--- a/tools/gn/config.h
+++ b/tools/gn/config.h
@@ -13,10 +13,10 @@
 class Config : public Item {
  public:
   Config(const Settings* settings, const Label& label);
-  virtual ~Config();
+  ~Config() override;
 
-  virtual Config* AsConfig() OVERRIDE;
-  virtual const Config* AsConfig() const OVERRIDE;
+  Config* AsConfig() OVERRIDE;
+  const Config* AsConfig() const OVERRIDE;
 
   ConfigValues& config_values() { return config_values_; }
   const ConfigValues& config_values() const { return config_values_; }