mac: Roll hermetic toolchain to Xcode 8.3.2.

build_overrides/build.gni

Index: build_overrides/build.gni
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 7b632bd2e648b9ca86096741a375487d0365a020..066298f1f76b3ab6b1250488de76930ddee0e31d 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -58,5 +58,8 @@ 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
 }