This reduces the number of indrections when accessing the Extensions
or Caps structures. It will provide a small speed-up to some methods.
It also cleans up the code.

Bug: angleproject:2966
Change-Id: Idddac70758c42c1c2b75c885d0cacc8a5c458685
Reviewed-on: https://chromium-review.googlesource.com/c/1392391
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Markus Tavenrath <matavenrath@nvidia.com>