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/255450Reviewed-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>