tools/gn: Pass clang-modernize -use-nullptr over it.

tools/gn/template.cc

Index: tools/gn/template.cc
diff --git a/tools/gn/template.cc b/tools/gn/template.cc
index 7a0dbdd52d7b320a855fc16948a99905ebb770c3..d7736cfa4f31152cc71d64479cfb8faf319ee09d 100644
--- a/tools/gn/template.cc
+++ b/tools/gn/template.cc
@@ -67,7 +67,7 @@ Value Template::Invoke(Scope* scope,
   // if we instead create a value and then set the scope on it, the copy can
   // be avoided.
   const char kInvoker[] = "invoker";
-  template_scope.SetValue(kInvoker, Value(NULL, scoped_ptr<Scope>()),
+  template_scope.SetValue(kInvoker, Value(nullptr, scoped_ptr<Scope>()),
                           invocation);
   Value* invoker_value = template_scope.GetMutableValue(kInvoker, false);
   invoker_value->SetScopeValue(invocation_scope.Pass());