This change adds ANGLE support for abs/sign variants introduced
in ESSL3.

BUG=angle:923
TEST=dEQP tests
Tests passing 100% because of this change:
dEQP-GLES3.functional.shaders.builtin_functions.common.abs
dEQP-GLES3.functional.shaders.builtin_functions.common.sign

Change-Id: If22032be2c1ed08451275262e311ef5ac613d45e
Reviewed-on: https://chromium-review.googlesource.com/251060Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>