Remove base level from texture storage. BUG=angle:596 Change-Id: I66173f77b6c5aed3b8836cd2c9f1da02ae769429 Reviewed-on: https://chromium-review.googlesource.com/192056 Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>