Fix Metal build from recent command buffer change

Bug: skia:
Change-Id: I93653a533499411118d3e55015d55000a465b51b
Reviewed-on: https://skia-review.googlesource.com/38401
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
1 file changed