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As Storage IDs are used by encrypted media to bind persistent licenses
to the device which is authorized to play the content, this updates the
preference so that it's authenticated and not easily modifiable via
plain text.

BUG=478960
TEST=unittests pass

Change-Id: I60c0a6a023f3532a6180859c753a1dd42f266c0e
Reviewed-on: https://chromium-review.googlesource.com/683309Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506875}