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Replace it with modern threading constructs:
 - PostTaskAndReplyWithResult gets rid of most of the logic FileThreadDeserializer was implementing.
 - The remainder logic didn't require any class state so it was moved to anonymous methods.

Also declare JsonPrefStore explicitly NonThreadSafe (the only actions outside the UI thread should happen by posting anonymous tasks to the |sequenced_task_runner_|).

This is a stepping stone in cleaning up JsonPrefStore to eventually get rid of PrefStore's ref-counting scheme.

BUG=393081

Review URL: https://codereview.chromium.org/400673008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284754 0039d316-1c4b-4281-b951-d872f2087c98