Use hermetic toolchain on Mac, except for local iOS builds

build_overrides/build.gni

 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # Some non-Chromium builds don't support building java targets.
 enable_java_templates = true
@@ skipping 17 matching lines @@
     rebase_path("//tools_webrtc/ubsan/vptr_blacklist.txt", root_build_dir)
 
 # Android lint suppressions file
 lint_suppressions_file = "//tools_webrtc/android/suppressions.xml"
 
 # For Chromium, Android 32-bit non-component, non-clang builds hit a 4GiB size
 # limit, making them requiring symbol_level=2. WebRTC doesn't hit that problem
 # so we just ignore that assert. See https://crbug.com/648948 for more info.
 ignore_elf32_limitations = true
 
-# Use bundled hermetic Xcode installation maintainted by Chromium.
-use_system_xcode = false
+# Use bundled hermetic Xcode installation maintainted by Chromium,
+# except for local iOS builds where it's unsupported.
+if (host_os == "mac") {
+  _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
+                        [ target_os ],
+                        "value")
+  assert(_result != 2,
+         "Do not allow building targets with the default" +
+             "hermetic toolchain if the minimum OS version is not met.")
+  use_system_xcode = _result == 0
+}