This is cleaner than indexing them by their name string.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I6cdc4125fadb6c7049ad8664e8a66da2a9a5bb8d
Reviewed-on: https://chromium-review.googlesource.com/836892
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>