Could cause a crash when running the WebGL CTS with Firefox.

BUG=angle:675

Change-Id: I58bf9eed622660d4702b775f368ff9cbd693197a
Reviewed-on: https://chromium-review.googlesource.com/203456Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>