Google's SafeBrowsing database is currently optimized for top-level loads. They no longer suggest running SafeBrowsing protocol on subresource loads. In this patch we introduce a pref to contorl the SafeBrowsing behavior on subresource loads. The propose is to follow the suggestion from Google to only run SafeBrowsing protocoal for top-level loads. Differential Revision: https://phabricator.services.mozilla.com/D236156