Use writeToSlot to update fSlots.

This gives us a natural foothold to write variable-change trace ops.

Change-Id: I0616ed374a3cf63ad33b6f14696acefde0741384
Bug: skia:12614
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/468826
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed