Using the same design as for the Framebuffer::Data helper, we can
use a struct to share between the object and the Impl. This also
gives the Impl access to the maxEnabledAttrib, and saves some
duplicated storage.

BUG=angleproject:1040
TEST=WebGL CTS, end2end_tests, unittests

Change-Id: I55c91e8a5f3dcae302cab441182320aafd5375ef
Reviewed-on: https://chromium-review.googlesource.com/283930Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>