ES 3.0 doesn't require non-constant sampler array indexing, and we
can't require it for ES 2 without breaking WebGL for some users.

BUG=671280

Change-Id: Ib04446284acc92410a90c683a3d222871de554a8
Reviewed-on: https://chromium-review.googlesource.com/416195
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>