This is cleaner than indexing them by their name string.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I0d0ef5e3f6a3f26c94f096b086cdf3da40d495e4
Reviewed-on: https://chromium-review.googlesource.com/845559
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>