Standardize usage of virtual/override/final 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 5b19aad6fc9c03f4fd7bba9f8b3c5922f5929e28..329a8f61074a781fd82197344a7628b466914c7c 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -52,15 +52,15 @@ 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; }