Make use of the buffer data null hint a GrContextOption

TBR=bsalomon@google.com
Change-Id: I5a3fd18479ca8c95e1bc8c087c28346264049eb0
Reviewed-on: https://skia-review.googlesource.com/111604
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed