macOS: Use system Xcode on 10.14.

A previous CL: https://chromium-review.googlesource.com/c/1333877 commented out
an assert to allow default fallback to the local toolchain, but failed to update
the subsequent conditional correctly.

Bug: 904400
Change-Id: I8d299e60645db4821dbb79c45b57ac1c01767d57
Reviewed-on: https://chromium-review.googlesource.com/c/1335949
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608359}

build_overrides/build.gni

@@ -56,5 +56,5 @@ if (host_os == "mac" && use_system_xcode == "") {
   # 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
+  use_system_xcode = _result != 1
 }