Fix ConvertPrecision. 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/206565 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>