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This adds possibility to enable/disable ARC and to view ARC storage
usage in case disable-arc-opt-in-verification. This was primarily used
in test before and now is used by some ARC devs for feature
prototyping.

TEST=Manually, corresponded sections appear in settings and work as
     expected.
BUG=b:120556416

Change-Id: I959bcabd729b76e6aa446d1fd85fe6c2b5f5660b
Reviewed-on: https://chromium-review.googlesource.com/c/1374433Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616086}