It's fully conformant to not support separate stencil masks in WebGL,
so we only need to print the error if we're failing validation from
the limitation.

BUG=angleproject:1523

Change-Id: Iaa08567180dee839a7b78e61e5a4ad7208c41924
Reviewed-on: https://chromium-review.googlesource.com/425488Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>