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 <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

src/libANGLE/renderer/metal/DisplayMtl.mm

diff --git a/src/libANGLE/renderer/metal/DisplayMtl.mm b/src/libANGLE/renderer/metal/DisplayMtl.mm
index 7debbe3..53e0419 100644
--- a/src/libANGLE/renderer/metal/DisplayMtl.mm
+++ b/src/libANGLE/renderer/metal/DisplayMtl.mm
@@ -22,11 +22,7 @@
 
 bool DisplayMtl::IsMetalAvailable()
 {
-    // We only support macos 10.13+ and iOS 11 for now. Since they are requirements for Metal 2.0.
-    if (@available(macOS 10.13, iOS 11, *))
-    {
-        return true;
-    }
+    UNIMPLEMENTED();
     return false;
 }