The extension spec dictates a format mismatch generates invalid
operation. Since we can't have the same internal format map to
different actual/native formats, this check is more strict and
we can replace the previous one.

BUG=angle:769

Change-Id: Ic1b5c335ba38a4f2feaffa1478a65e7fcf7f75ea
Reviewed-on: https://chromium-review.googlesource.com/221065Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>