This optimizes some of our dynamic switching. It uses packed enum maps.

Bug: angleproject:2747
Change-Id: Ibd2f9306d066f2fd9eb64c99a25668b7ba5c009c
Reviewed-on: https://chromium-review.googlesource.com/1171505
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>