Move SkSurface::MakeNull to SkSurfaces::Null

Follow-up to https://skia-review.googlesource.com/c/skia/+/687639

Change-Id: I19c01a0c3957460d6c79c9edb2ddabd4c8c2d2cf
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/696537
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
8 files changed