Using a purpose-built class for function lookups instead of using
a combination of TFunction and a struct container for the this node
and arguments makes the code clearer.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I3f345d836abeaa7f84cc46b4b840fd06c7e2e1a7
Reviewed-on: https://chromium-review.googlesource.com/897363Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>