This allow for predeclaring the enum. It solves some include dependency
issues.

Bug: angleproject:2729
Change-Id: Ibbbab0796e466c62848404ba277c5f454fd9ac62
Reviewed-on: https://chromium-review.googlesource.com/1142299
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>