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

tools/gn/function_get_target_outputs.cc

Index: tools/gn/function_get_target_outputs.cc
diff --git a/tools/gn/function_get_target_outputs.cc b/tools/gn/function_get_target_outputs.cc
index dd2de0c733cbbb196989ede747c820dab89ce93e..6e3cf6563ca0dde20ca4c359c5b95ec2a4763ab5 100644
--- a/tools/gn/function_get_target_outputs.cc
+++ b/tools/gn/function_get_target_outputs.cc
@@ -86,7 +86,7 @@ Value RunGetTargetOutputs(Scope* scope,
   // Find the referenced target. The targets previously encountered in this
   // scope will have been stashed in the item collector (they'll be dispatched
   // when this file is done running) so we can look through them.
-  const Target* target = NULL;
+  const Target* target = nullptr;
   Scope::ItemVector* collector = scope->GetItemCollector();
   if (!collector) {
     *err = Err(function, "No targets defined in this context.");