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

tools/gn/label.cc

Index: tools/gn/label.cc
diff --git a/tools/gn/label.cc b/tools/gn/label.cc
index f672a3a23c2f5265f7b50ee63f44b6a3bc04d8b9..65fcef82d759bb00a57a9d95ef4ebde53ea04e7a 100644
--- a/tools/gn/label.cc
+++ b/tools/gn/label.cc
@@ -177,9 +177,9 @@ bool Resolve(const SourceDir& current_dir,
       *out_toolchain_name = current_toolchain.name();
       return true;
     } else {
-      return Resolve(current_dir, current_toolchain,
-                     original_value, toolchain_piece,
-                     out_toolchain_dir, out_toolchain_name, NULL, NULL, err);
+      return Resolve(current_dir, current_toolchain, original_value,
+                     toolchain_piece, out_toolchain_dir, out_toolchain_name,
+                     nullptr, nullptr, err);
     }
   }
   return true;