Permits depth-stencil attachment points for WebGL.

BUG=angleproject:2090

Change-Id: I7f5a7c63f2a4a76116ce5639833e5fd8d7f50ffb
Reviewed-on: https://chromium-review.googlesource.com/764591Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>