D3D11: Fix Array texture depth-stencil attachments. We were missing support for these layer depth-stencil attachments. This fixes running a WebGL test for this feature. BUG=angleproject:1260 BUG=angleproject:1315 Change-Id: Ida7d44ba1fe65d5da95d2db721e497bfa685db77 Reviewed-on: https://chromium-review.googlesource.com/327402 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>