Custom TLD lists (added in #17748), may contain domain of the 3-d level,
however builtin TLD lists does not have such records, so it is not
affected.
Note that this will significantly increase hashtable lookups.
Fixes: #17748
v2: Add a note that top_level_domains_lists aren not applied w/o restart
v3: Remove ExtractFirstSignificantSubdomain{Default,Custom}Lookup.h headers
v4: TLDListsHolder: remove FIXME for dense_hash_map (this is not significant)