This will be accessible to the Impl, so they won't have to store a
copy of the pointer.

BUG=angleproject:1319

Change-Id: I298a11a243d3d32f4c885273e55162e6862f3e16
Reviewed-on: https://chromium-review.googlesource.com/419697
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>