Removes UNREACHABLE in display initialization As it's possible for metal to be unavailable, UNREACHABLE() should not be used. Chrome has started shipping some builds with asserts enabled, so that it's possible for the gpu to crash here. Removing the UNREACHABLE makes it so the code doesn't crash and gpu fallback happens. Bug: chromium:1354718 Change-Id: I613c44b428c1de73b4b9937505e151d9416388ef Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4437615 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Scott Violet <sky@chromium.org>