Convert OVERRIDE -> override and update virtual/final usage 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 9f3d19aedb51e62b9d668f5dc9f50c4d0cc46211..c94d8ed5505bf585c666ee93f59cd020ab9796b4 100644
--- a/tools/gn/toolchain.h
+++ b/tools/gn/toolchain.h
@@ -5,7 +5,6 @@
 #ifndef TOOLS_GN_TOOLCHAIN_H_
 #define TOOLS_GN_TOOLCHAIN_H_
 
-#include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
@@ -61,11 +60,11 @@ 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);