Revert of Improve computation of PrefHashCalculator::legacy_device_id_instance_.

Revert of Improve computation of PrefHashCalculator::legacy_device_id_instance_. (https://codereview.chromium.org/231673002/)

Reason for revert:
PrefHashCalculatorTest.TestLegacyDeviceIdAlgorithm is failing in some configurations.

Original issue's description:
> Improve computation of PrefHashCalculator::legacy_device_id_instance_.
> 
> 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
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=262995

TBR=erikwright@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=361687
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=263028

[gab, 2014-04-10 18:02:09]

Created Revert of Improve computation of
PrefHashCalculator::legacy_device_id_instance_.

[commit-bot: I haz the power, 2014-04-10 18:02:11]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/gab@chromium.org/228763007/1

[commit-bot: I haz the power, 2014-04-10 18:03:27]

Change committed as 263028