Also add a small fix for a WebGL test where DrawElements with a count
of 0 should skip OOB checks.

BUG=angleproject:1523
BUG=chromium:668223

Change-Id: I67fbe939f6c8b925551b658b6178dbbae982df89
Reviewed-on: https://chromium-review.googlesource.com/435279Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>