Reland "Delete deprecated alpha threshold header"

This reverts commit 140adc67621956d787c5580b2fc23a4fb8fba31a.

Reason for revert: sksl issue addressed in https://skia-review.googlesource.com/c/skia/+/368813

Original change's description:
> Revert "Delete deprecated alpha threshold header"
>
> This reverts commit 544118863781f84b53c537335d94febe59f8e150.
>
> Reason for revert: this couldn't really be breaking angle-only sksl compilation, right? Let's find out!
>
> Original change's description:
> > Delete deprecated alpha threshold header
> >
> > Bug: skia:11230
> > Change-Id: Icd927cfdcca71a48dce16bcd6c40489dad92a259
> > Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368238
> > Reviewed-by: Brian Salomon <bsalomon@google.com>
> > Reviewed-by: Robert Phillips <robertphillips@google.com>
> > Commit-Queue: Michael Ludwig <michaelludwig@google.com>
>
> TBR=bsalomon@google.com,robertphillips@google.com,reed@google.com,michaelludwig@google.com
>
> Change-Id: Ie5b51e6cca39acb34dbf7b1e702209aba69ea437
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:11230
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368798
> Reviewed-by: Michael Ludwig <michaelludwig@google.com>
> Commit-Queue: Michael Ludwig <michaelludwig@google.com>

TBR=bsalomon@google.com,robertphillips@google.com,reed@google.com,michaelludwig@google.com

# Not skipping CQ checks because this is a reland.

Bug: skia:11230
Change-Id: I6d5bb2f42edd73f5ee4e121aaaff2f348661cf18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369056
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
6 files changed