This enum is queryable in ES3 or with the GL_OES_standard_derivatives
extension.

BUG=2110

Change-Id: I688777bcc65468d796333a24223d24cc66170788
Reviewed-on: https://chromium-review.googlesource.com/576156Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>