SafeBrowsing prefix files LOAD/SAVE operations are handled in xxxPrefixSet.cpp. It would be more clear if xxxPrefixSet.cpp only processes prefix data, while LookupCacheV2/LookupCacheV4 which use prefix set process file. This patch doesn't change any behavior, testcases need to update because the LookupCache & xxxPrefixSet APIs are changed. Differential Revision: https://phabricator.services.mozilla.com/D21462