validate GrMtlBuffer after forcing it to be static

Bug: skia:
Change-Id: Iaa9ac00f8533414bb81ab87eefbc2b035bcf1a53
Reviewed-on: https://skia-review.googlesource.com/145328
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
1 file changed