Add a class for function lookups 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/897363 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>