This state can share vital pieces of information with the impl.
The most elementary state is the buffer size, which then the
impl doesn't need to replicate.

BUG=angleproject:1579

Change-Id: I341393c64a6e49de65c1d53b1bad2fa143209862
Reviewed-on: https://chromium-review.googlesource.com/406644Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>