Revert "Reland "Revert "Reland "GPU support for SkMixers""""

This reverts commit 1263889f3a3ea44db959f043254f4ee9e4014ac9.

Bug: skia:
Change-Id: I647cb228e4a8169f12f4afadaf1db06843e4e5fd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206391
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed