Right now, now matter which SafeBrowsing protocol we use (V2 or V4), we always use variable-length prefix set to store prefixes. Since nsUrlClassifierPrefixSet is a member of VariableLengthPrefixSet and it is never used alone, we can remove nsIMemoryReporter from it to prevent calculating the same memory report twice. Differential Revision: https://phabricator.services.mozilla.com/D81171