Expose the already existing PrefStore implementations, which are
owned through the profile, through the new prefs service. This
new prefs service will replace the current pref service prototype
used in mash and support synchronous deletions and decoupling the
creation of the underling pref stores.

The service is gated by a default-off feature flag.

BUG=654988

Review-Url: https://codereview.chromium.org/2740493002
Cr-Commit-Position: refs/heads/master@{#455968}