Convert GrPorterDuff test from NullGL to Mock context

Bug: skia:
Change-Id: Id403aabcae1dbf38e55381bb3b4353e85a12a745
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203705
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed