TEST=conformance2/rendering/instanced-arrays

BUG=angleproject:1988

Change-Id: Ie513dcc7b1af540764fd5fe3790d3e6e3457f048
Reviewed-on: https://chromium-review.googlesource.com/475136Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>