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/327402Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>