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There are two methods in this class taking a callback.

 - StartQueryInfo stores the callback in a queue and then calls them
   when a system query is done. The callbacks are always cleared after
   calling so these can be made OnceCallback.

 - InitializeProvider simply calls the given Closure and the only
   override in StorageInfoProvider passes it to
   StorageMonitor::EnsureInitialized which takes it as a OnceClosure so
   this is also safe to make OnceClosure.

And we remove GetStorageFreeSpaceCallback since it's no longer used
anywhere.

Bug: 1152268
Change-Id: Ifa05987ef60c57046df4f1ee4753957a061e0edb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597575Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839117}