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

tools/gn/target.h

Index: tools/gn/target.h
diff --git a/tools/gn/target.h b/tools/gn/target.h
index b949978a012bec1da4d6ade0219782b05b845a47..8e5a3e283ca52e5668a258d98713b8a9f81e8bd8 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -10,7 +10,6 @@
 #include <vector>
 
 #include "base/basictypes.h"
-#include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
@@ -52,15 +51,15 @@ class Target : public Item {
   typedef std::vector<std::string> StringVector;
 
   Target(const Settings* settings, const Label& label);
-  virtual ~Target();
+  ~Target() override;
 
   // Returns a string naming the output type.
   static const char* GetStringForOutputType(OutputType type);
 
   // Item overrides.
-  virtual Target* AsTarget() override;
-  virtual const Target* AsTarget() const override;
-  virtual bool OnResolved(Err* err) override;
+  Target* AsTarget() override;
+  const Target* AsTarget() const override;
+  bool OnResolved(Err* err) override;
 
   OutputType output_type() const { return output_type_; }
   void set_output_type(OutputType t) { output_type_ = t; }