Fix this by consistently checking if the attachment is enabled when we
update the serials.

Also includes a regression test and more ASSERTs.

Bug: angleproject:4540
Change-Id: I154d23cad71f1674d893390f923f45c643a58925
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2134409
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>