When a line loop is drawn with byte indices from a buffer, translate
to short.  Enable test.

BUG=angleproject:2802

Change-Id: Ie178ec866387be85b91291c28e1978db0a09c20a
Reviewed-on: https://chromium-review.googlesource.com/1237293Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>