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Although PolicyBlocklistNavigationThrottle uses Safe Sites, blocklists
and Safe Sites are conceptually different. Also, Safe Sites does not
require Prefs, which makes it easier to use outside //chrome.

Also extract SafeSearchService from PolicyBlocklistService.

Bug: 1147231
Change-Id: I4e2714478177bebcf26a07ed0e6230aa67e0792f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530145
Auto-Submit: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826798}