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Part of the ongoing work to cleanup the filesystem callbacks to be OnceCallback
where possible.

No logic changes.

Bug: 875700
Change-Id: I69c5518eaee657e0ce42d8bde3687696fa9047b9
Reviewed-on: https://chromium-review.googlesource.com/1217644Reviewed-by: Anand Mistry <amistry@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590152}