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>
6 files changed