Move the DDL unreffing to task destructor and update tests

Bug: skia:10425
Change-Id: If7f7bdfb21d4c3eefcb7d1a0d68e590f6bf97b0f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299384
Commit-Queue: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed