This method had never been fully implemented, and was returning
"highp" for every case value.

This should fix problems with the GPU FYI bots.

BUG=angle:691

Change-Id: Ie6ffa80cf2c063e506df06fb25d622227edeffa3
Reviewed-on: https://chromium-review.googlesource.com/206565Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>