We can validate to avoid triggering driver issues even if the native
GLES spec is not telling us to.

This will fix WebGL in Chromium when it's being run with the
--use-cmd-decoder=validating --use-angle=gl config.

BUG=chromium:830046
TEST=WebGL conformance tests

Change-Id: I2d61182cb6cbe46b52e1d9b7ed6b4035defee082
Reviewed-on: https://chromium-review.googlesource.com/1033743
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>