Fill in missing GrMtlCaps functions and fix for 0 buffer alloc. Bug: skia: Change-Id: I4981e49dc45d607f3f6493da67844b6dfa74a551 Reviewed-on: https://skia-review.googlesource.com/c/189493 Commit-Queue: Greg Daniel <egdaniel@google.com> Reviewed-by: Jim Van Verth <jvanverth@google.com>