Metal: Disable fence sync extension on Nvidia
Metal: Disable fence sync extension on Nvidia

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 <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Reviewed-by: Kai Ninomiya <3e985eef1b20102024c2478684a0e52d76500ed0@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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