[task scheduler] Use cleanup.AtExit to close the DB
[task scheduler] Use cleanup.AtExit to close the DB

Maybe this would help with the DB problem we saw last week.

Bug: skia:
Change-Id: I12db1dc0f3b80314f0359ecef2ee4fccef7c9b40
Reviewed-on: https://skia-review.googlesource.com/107300
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

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