[graphite] Distribute key creation to SkShaders

Bug: skia:12701
Change-Id: I33d24fc319acbfbb3a218a8dd916c5a64f15f028
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503342
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
19 files changed