Remove remaining references to GrSlug

Chrome was migrated in https://crrev.com/c/4605336

Change-Id: I15012449d10f92782cf311b4e7086584372d5f2b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/710677
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
8 files changed