[FileSystem] Change callback types in FileSystemOperation to be Once or Repeating.

Bug: 714018
Change-Id: I02bc49853ff630a0422ba0ddc8f08b7412046327
Reviewed-on: https://chromium-review.googlesource.com/1255148
Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595636}
23 files changed