[graphite] Add iOS compile bots

Bug: skia:12561
Change-Id: I0a739dcfd4214ee4855f04a70fb6f66ab638bf2e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/461478
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed