Report Metal as unavailable
Report Metal as unavailable

Metal backend is not ready yet, so report as unavailable
to avoid DisplayMtl being created by frontend.

Bug: angleproject:2634
Change-Id: Ia45977cede01f62347967eefe661b63bef3575e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1865736
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle