Add UMA histograms for tracking empty usernames.

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 4c65080b080dab0fcbc0947e8b477e872fcd4349..c04a3e92ebccb895493dda1938b33766d64c5719 100644
--- a/components/password_manager/core/browser/login_database.cc
+++ b/components/password_manager/core/browser/login_database.cc
@@ -454,6 +454,14 @@ void LoginDatabase::ReportMetrics(const std::string& sync_username,
   UMA_HISTOGRAM_ENUMERATION("PasswordManager.SyncingAccountState",
                             2 * sync_username.empty() + syncing_account_saved,
                             4);
+
+  sql::Statement empty_usernames_statement(db_.GetCachedStatement(
+      SQL_FROM_HERE, "SELECT COUNT(*) FROM logins "
+                     "WHERE blacklisted_by_user=0 AND username_value=''"));
+  if (empty_usernames_statement.Step()) {
+    int empty_forms = empty_usernames_statement.ColumnInt(0);
+    UMA_HISTOGRAM_COUNTS_100("PasswordManager.NumEmptyUsernames", empty_forms);
+  }
 }
 
 PasswordStoreChangeList LoginDatabase::AddLogin(const PasswordForm& form) {