Fix derivatives being related to an extension on ESSL3 dFdx, dFdy and fwidth are parts of the core language in ESSL3. This fixes a regression from the patch "Set symbol extension at insertion". BUG=angle:926 Change-Id: I6c0bbce3afad1f064ff115a16e23ea79bd584e49 Reviewed-on: https://chromium-review.googlesource.com/255450 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com>