We had accidentally marked 'smoooth', 'centroid' and 'case'
as reserved in ES2, when they are available as identifiers.
Additionally 'sampler2DShadow' was marked as free, instead
of reserved.

BUG=angle:550

Change-Id: I33ec1b790ad5b4d9a6f4d2ea2e91cec1ca8e7320
Reviewed-on: https://chromium-review.googlesource.com/186970Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>