This CL adds base::IgnoreReuse() and applies it to ImportantFileWriter.

In some rare case such as ImportantFileWriter, ServiceWorkerStorage, and
Mojo stuff, a called-only-once callback object is copied and passed to
separate code paths, that blocks OnceCallback migration.
base::IgnoreReuse() is a workaround for that by wrapping OnceCallback
into a RepeatingCallback, and ignores past-first calls.

BUG=668014

Review-Url: https://codereview.chromium.org/2820993002
Cr-Commit-Position: refs/heads/master@{#465979}