Fixed SkSL dFdy rehydrate test

dFdy wasn't working due to the DSL never having been started.

Bug: skia:12985
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/513938
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Change-Id: Ia73accea9565863287f841b029032269510b3af0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/522990
2 files changed