This allows us to stop duplicating some information in the impl.

BUG=angleproject:1635

Change-Id: If8f7d2418571c3254729f48c463814ec18ed2644
Reviewed-on: https://chromium-review.googlesource.com/469153
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>