Replace scoped_ptr with std::unique_ptr in //components/syncable_prefs

[email protected], [email protected], [email protected]
BUG=554298

Review URL: https://codereview.chromium.org/1913583002

Cr-Commit-Position: refs/heads/master@{#389276}
6 files changed