Add deprecation warning when running Metal on older OSes.

Older versions of MacOS are unstable when compiling shaders, and we
don't have the testing support for older versions of iOS.

Bug: skia:10777
Change-Id: I95d9fe0f1007af6bec5c7525dacda565e32989f2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320260
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed