[graphite] Use PaintParams::toKey in ExtractPaintData

Bug: skia:12701
Change-Id: I9af3c437512f2538066ddbbe576739c8873d0bc0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503345
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed