made GrMtlBuffer compatible with iOS

also fixed the fact that we were actually creating managed buffers at times.

Bug: skia:
Change-Id: I7ffe12b5decc8bceee30d671e1ab3cc30f7fad99
Reviewed-on: https://skia-review.googlesource.com/146170
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed