The following tests are flaky on Mac FYI Retina Release (NVIDIA), so the
Metal fence sync extension is being disabled for Nvidia.

Bug: chromium:1136673
Change-Id: I7e99f558950c6e867318e27f27bb526b4c209fcc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2463516
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>