Report the number of empty usernames without a nonempty pair.

components/password_manager/core/browser/login_database.cc

Index: components/password_manager/core/browser/login_database.cc
diff --git a/components/password_manager/core/browser/login_database.cc b/components/password_manager/core/browser/login_database.cc
index ab8d18f5a08b279dfd95b4a08286d4bc69219b30..f59f8c6d17a1cdc1797354bfd64b8a2792b15b55 100644
--- a/components/password_manager/core/browser/login_database.cc
+++ b/components/password_manager/core/browser/login_database.cc
@@ -504,6 +504,19 @@ void LoginDatabase::ReportMetrics(const std::string& sync_username,
     UMA_HISTOGRAM_COUNTS_100("PasswordManager.EmptyUsernames.CountInDatabase",
                              empty_forms);
   }
+
+  sql::Statement standalone_empty_usernames_statement(db_.GetCachedStatement(
+      SQL_FROM_HERE, "SELECT COUNT(*) FROM logins a "
+                     "WHERE a.blacklisted_by_user=0 AND a.username_value='' "
+                     "AND NOT EXISTS (SELECT * FROM logins b "
+                     "WHERE b.blacklisted_by_user=0 AND b.username_value!='' "
+                     "AND a.signon_realm = b.signon_realm)"));
+  if (standalone_empty_usernames_statement.Step()) {
+    int num_entries = standalone_empty_usernames_statement.ColumnInt(0);
+    UMA_HISTOGRAM_COUNTS_100(
+        "PasswordManager.EmptyUsernames.WithoutCorrespondingNonempty",
+        num_entries);
+  }
 }
 
 PasswordStoreChangeList LoginDatabase::AddLogin(const PasswordForm& form) {