Remove ARC from Metal backend

Change-Id: I5ab28f6eda3b37d1b82c94c7cc6eaa2ce59157da
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311113
Reviewed-by: Adlai Holler <adlai@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
40 files changed