@kamil explains this well in D11887. Most calls to unshimClientDB (all except for the new one introduced in that diff) get passed an older version of the Redux state, where cookie is a top-level field instead of being inside the KeyserverStore.
This diff changes the default behavior of unshimClientDB so that it clears that top-level field, instead of calling handleReduxMigrationFailure, which assumes that the cookie field has been migrated to inside the KeyserverStore.
Passing this as a third param might not be the most ideal way of doing this, but it's consistent with how we do it elsewhere.