Use new span APIS (time to dogfood)

(need this if we want to test clients w/o the guard)

Change-Id: I53a35d9582f62999e1f301a2c7e8ea5697b29cb9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1003783
Reviewed-by: Daniel Dilan <danieldilan@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>