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..6bfd9f222f71837f24a8d07a61e5035c2484d32a 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 "

[vasilii, 2015/04/15 15:51:55]

I'd reverse the condition as "There is a corresponding credential with non-empty
username". Thus, it's definitely a bug in the password manager. While a
standalone empty username can mean anything.

[msramek, 2015/04/16 14:34:26]

Well, that was my original intention. But then I realized that the histograms
CountInDatabase and "PairedWithNonempty" will not be well readable together.
Sure, if "PairedWithNonempty" has almost the same distribution as
CountInDatabase, we know that this is the overwhelming cause. But if it's, for
example,

CountInDatabase PairedWithNonempty
2: 1 user       1: 1 user  
3: 1 user       2: 1 user  

we won't be able to tell if only one or both of the people encountered an empty
password form.

I think this solution gives us a more interesting information, "How many empty
usernames (per user) are there, that we do not know the cause for yet?"

and the goal of this investigation is to define a histogram that has flat
zeroes.

+                     "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."
+                             "NotPairedWithNonempty",

[Ilya Sherman, 2015/04/15 22:56:05]

nit: Rather than splitting this string across multiple lines, please just wrap
the arguments, i.e.

    UMA_HISTOGRAM_COUNTS_100(
      "PasswordManager.EmptyUsernames.NotPairedWithNonempty", num_entries);

[msramek, 2015/04/16 14:34:26]

Done.

+                             num_entries);
+  }
 }
 
 PasswordStoreChangeList LoginDatabase::AddLogin(const PasswordForm& form) {