We can replace this by doing the same calculation in the HLSL-side
code.

BUG=angle:466

Change-Id: Iecae4a92e9037e851419ce73e6267094ee8071a2
Reviewed-on: https://chromium-review.googlesource.com/207251Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>