mac: Roll hermetic toolchain to Xcode 8.3.2.

build_overrides/build.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.
 
 # Several dependencies of Chrome require a different min sdk when built
 # as stand-alone projects. If this is ever not the case, these variables
 # can be removed.
 # See https://bugs.chromium.org/p/webrtc/issues/detail?id=5453.
 mac_sdk_min_build_override = "10.10"
 
@@ skipping 40 matching lines @@
   # instead use a hermetic install of Xcode. [The hermetic install can be
   # obtained with gclient sync after setting the environment variable
   # FORCE_MAC_TOOLCHAIN].
   use_system_xcode = ""
 }
 
 if (use_system_xcode == "") {
   _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
 }