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

tools/gn/function_get_target_outputs_unittest.cc

Index: tools/gn/function_get_target_outputs_unittest.cc
diff --git a/tools/gn/function_get_target_outputs_unittest.cc b/tools/gn/function_get_target_outputs_unittest.cc
index be46380c39d91b3fb5a71c378b3728e372a1814d..cd19d9956663bfc6d1def283e907bbb57cbebed6 100644
--- a/tools/gn/function_get_target_outputs_unittest.cc
+++ b/tools/gn/function_get_target_outputs_unittest.cc
@@ -20,7 +20,7 @@ class GetTargetOutputsTest : public testing::Test {
   Value GetTargetOutputs(const std::string& name, Err* err) {
     FunctionCallNode function;
     std::vector<Value> args;
-    args.push_back(Value(NULL, name));
+    args.push_back(Value(nullptr, name));
     return functions::RunGetTargetOutputs(setup_.scope(), &function, args, err);
   }