Add a variable use_system_xcode to GN.

chrome/BUILD.gn

 # Copyright 2014 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/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//build/config/locales.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/config/win/console_app.gni")
 import("//build/config/win/manifest.gni")
+import("//build/toolchain/toolchain.gni")
 import("//chrome/chrome_paks.gni")
 import("//chrome/common/features.gni")
 import("//chrome/process_version_rc_template.gni")
 import("//third_party/widevine/cdm/widevine.gni")
 import("//tools/resources/generate_resource_whitelist.gni")
 import("//v8/gni/v8.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 } else if (is_mac) {
@@ skipping 1004 matching lines @@
     action("verify_chrome_framework_order") {
       script = "//chrome/tools/build/mac/run_verify_order.py"
       stamp_file = "$target_out_dir/run_$target_name.stamp"
       inputs = [
         script,
         "//chrome/tools/build/mac/verify_order",
       ]
       args = [
         "--stamp",
         rebase_path(stamp_file, root_out_dir),
+      ]
+      if (!use_system_xcode) {
+        args += [
+          "--developer-dir",
+          hermetic_xcode_path,
+        ]
+      }
+      args += [
         "_ChromeMain",
         rebase_path(
             "$root_out_dir/$chrome_framework_name.framework/$chrome_framework_name",
             root_out_dir),
       ]
       outputs = [
         stamp_file,
       ]
       public_deps = [
         ":chrome_framework",
@@ skipping 477 matching lines @@
       "//chrome/app/theme/$branding_path_component/product_logo_48.png",
       "//chrome/tools/build/linux/chrome-wrapper",
       "//third_party/xdg-utils/scripts/xdg-mime",
       "//third_party/xdg-utils/scripts/xdg-settings",
     ]
     outputs = [
       "$root_out_dir/{{source_file_part}}",
     ]
   }
 }