This will refactor will help use packed enums for buffer targets.

BUG=angleproject:2169

Change-Id: Ie7ed3e105f89457c67027e6598d7e29503ad355c
Reviewed-on: https://chromium-review.googlesource.com/745181
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>