Also add a warning on top of the UnrollFlatten test that it works only when HLSL is enabled. BUG=angleproject:1140 Change-Id: Ide961bbed70dc9ed2a604f13273e4209b1dec29b Reviewed-on: https://chromium-review.googlesource.com/295125Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>
