Remove support for iOS12

Change-Id: I65e8dfabfdb907396808ad3c8e3deebdc568ecfa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1002871
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
9 files changed