1. Extract SupervisedUserAsyncURLChecker and its unittest to a separate
directory to make it reusable. Rename it to SafeSearchURLChecker
as it's now available not only to supervised users, and as its
asynchronicity does not need to be stressed (as there is no synchronous
counterpart).

2. Decople it from the suprevised_users/ code; namely, introduce a
SafeSearchURLChecker::Classification enum that has a direct mapping to
SupervisedUserURLFilter::FilteringBehavior instead of using
FilteringBehavior directly.

3. Update the callsites in supervised_users/, headers, etc. No functional
changes to the code have been made in this CL.

BUG=653479

Review-Url: https://codereview.chromium.org/2399823002
Cr-Commit-Position: refs/heads/master@{#424149}