Migrate AcquireFileAccessPermissionCallback to OnceCallback.

Bug: 714018
Change-Id: I73cf48ba17dd96f6f4571bad941b6669509ba6b2
Reviewed-on: https://chromium-review.googlesource.com/1117575
Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571927}
8 files changed