GrAlphaThresholdFragmentProcessor uses child processor for mask.

Bug: skia:10139

Change-Id: I526f8ca77de0ed6ab1fe16b22201043ebf5c3716
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296477
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed