Convert OVERRIDE -> override and update virtual/final usage in tools/gn/.

tools/gn/config.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_CONFIG_H_
 #define TOOLS_GN_CONFIG_H_
 
-#include "base/compiler_specific.h"
+#include "base/macros.h"
 #include "tools/gn/config_values.h"
 #include "tools/gn/item.h"
 
 // Represents a named config in the dependency graph.
 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_; }
 
  private:
   ConfigValues config_values_;
 
   DISALLOW_COPY_AND_ASSIGN(Config);
 };
 
 #endif  // TOOLS_GN_CONFIG_H_