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

tools/gn/toolchain.h

Index: tools/gn/toolchain.h
diff --git a/tools/gn/toolchain.h b/tools/gn/toolchain.h
index 393596cb2917f7d11832f0412307cca078939214..43dd5192b04e8b2ed277cb45fba4030cea0108d4 100644
--- a/tools/gn/toolchain.h
+++ b/tools/gn/toolchain.h
@@ -61,11 +61,11 @@ class Toolchain : public Item {
   static const char* kToolCopy;
 
   Toolchain(const Settings* settings, const Label& label);
-  virtual ~Toolchain();
+  ~Toolchain() override;
 
   // Item overrides.
-  virtual Toolchain* AsToolchain() OVERRIDE;
-  virtual const Toolchain* AsToolchain() const OVERRIDE;
+  Toolchain* AsToolchain() OVERRIDE;
+  const Toolchain* AsToolchain() const OVERRIDE;
 
   // Returns TYPE_NONE on failure.
   static ToolType ToolNameToType(const base::StringPiece& str);