This will make it easier to add more built-in functions.

BUG=angleproject:1730
TEST=angle_unittests

Change-Id: I5219a83e964b6ee15c690e108313960a873dd8e5
Reviewed-on: https://chromium-review.googlesource.com/431031Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>