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As per the attached bug, replace usage of Replace (bool, const std::string&) in
filesystem_api_util.cc with base::Optional<std::string> for code health.

Also, change from RepeatingCallback to OnceCallback for all filesystem_api_util
defined methods.

Also, fix linter warnings.

There are no logic changes.

Bug: 729442, 875700
Change-Id: Ib27634b9ba1372cb007ba6a190608ec4128bbeb3
Reviewed-on: https://chromium-review.googlesource.com/1215425Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589842}