Add a variable use_system_xcode to GN.

chrome/app/nibs/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/mac/rules.gni")
 
 assert(is_mac)
 
 translated_xibs = [
   "AppMenu.xib",
@@ skipping 49 matching lines @@
 }
 
 action("localizer_table") {
   script = "generate_localizer.py"
 
   sources = translated_xibs
   table_path = "$target_gen_dir/localizer_table.h"
   outputs = [
     table_path,
   ]
-  args = [ rebase_path(table_path, root_build_dir) ] +
-         rebase_path(translated_xibs, root_build_dir)
+  args = [
+    "--output_path",
+    rebase_path(table_path, root_build_dir),
+  ]
+  if (!use_system_xcode) {
+    args += [
+      "--developer_dir",
+      hermetic_xcode_path,
+    ]
+  }
+  args += rebase_path(translated_xibs, root_build_dir)
 }