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1) Compute it at most once in the lifetime of the browser (rather than at most once per PrefHashCalculator).

2) Explicitly AllowIO in the scope of this computation. This isn't ideal, but there is no other good way until we can get rid of this legacy artifact in a few milestones.

Note that I have to skip a presubmit error for (2): "Banned functions were used. New code should not use ScopedAllowIO." :(.

BUG=361687
R=erikwright@chromium.org

Review URL: https://codereview.chromium.org/231673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262995 0039d316-1c4b-4281-b951-d872f2087c98