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This patch removes use of filesystem's VoidCallback.  The
motivation is the same as https://crrev.com/844307 .

Bug: 778580
Change-Id: I2f57919461a31a4f64af287b40e0a3a6d1f242cb
Reviewed-on: https://chromium-review.googlesource.com/888234
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532414}