Check crostini deleted paths on threadpool, not IO thread

I erroneously used the IO thread for checking if paths exist
when first creating this code.

Change-Id: I2f0180be7e4ad7caec4d11a28bf3e434533a6e94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619444
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661274}
2 files changed