- Revise entry point definitions to use packed enums

BUG=angleproject:2306

Change-Id: I06ad95f475d1dbaf07ec24ff2544503c4a44e826
Reviewed-on: https://chromium-review.googlesource.com/1090996Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Lingfeng Yang <lfy@google.com>