This test is ported from webgl test element-index-uint.html.

BUG=angleproject:1393, angleproject:1463

Change-Id: I165e3dd2913968c8cc035c570a7bcaf91aed095a
Reviewed-on: https://chromium-review.googlesource.com/651239
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>