We were using DEPTH_STENCIL instead of DEPTH_STENCIL_ATTACHMENT.

BUG=angle:739

Change-Id: I0c746c374577dca2b965dfd7ccfac793f7075efb
Reviewed-on: https://chromium-review.googlesource.com/218592Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>