The test can fail if it uses a config where the default framebuffer does
not have a stencil buffer.

BUG=angle:658

Change-Id: I82eaf5b508182514439e69469e77591e1aceaf66
Reviewed-on: https://chromium-review.googlesource.com/246461Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>