Under certain situations an integer overflow could lead to ANGLE
writing to places where it shouldn't.

BUG=518206

Change-Id: I9217685daecb160a4072fbf79c26e5bee9f4621e
Reviewed-on: https://chromium-review.googlesource.com/292391Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>