Add a variable use_system_xcode to GN.

build/config/mac/base_rules.gni

 # 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.
 
 # This file contains rules that are shared between Mac and iOS.
 
 import("//build/toolchain/toolchain.gni")
 import("//build/config/mac/symbols.gni")
 
 if (is_mac) {
@@ skipping 137 matching lines @@
     ibtool_flags = []
     if (defined(invoker.ibtool_flags)) {
       ibtool_flags = invoker.ibtool_flags
     }
 
     script = "//build/config/mac/compile_xib.py"
     sources = invoker.sources
     outputs = [
       "$target_gen_dir/$target_name/{{source_name_part}}.nib",
     ]
-    args =
-        [
-          "--input",
-          "{{source}}",
-          "--output",
-          rebase_path("$target_gen_dir/$target_name/{{source_name_part}}.nib"),
-        ] + ibtool_flags
+    args = [
+      "--input",
+      "{{source}}",
+      "--output",
+      rebase_path("$target_gen_dir/$target_name/{{source_name_part}}.nib"),
+    ]
+    if (!use_system_xcode) {
+      args += [
+        "--developer_dir",
+        hermetic_xcode_path,
+      ]
+    }
+    args += ibtool_flags
   }
 }