prefs: Replace base::MakeUnique with std::make_unique

Bug: 755727
Change-Id: I64bb34f8807685ee0bc9fe581bffee9e3f5fa031
Reviewed-on: https://chromium-review.googlesource.com/846584
Commit-Queue: Jinho Bang <[email protected]>
Reviewed-by: Dominic Battré <[email protected]>
Cr-Commit-Position: refs/heads/master@{#526428}
13 files changed