Restricting the enum to uint8_t and reorder type for better alignment.

Bug: v8:7926
Change-Id: Ib37d600a189d12bac3c6aa8cf4b88970f8547548
Reviewed-on: https://chromium-review.googlesource.com/1188125
Commit-Queue: Florian Sattler <sattlerf@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55388}