Add a text/password field count metric for empty username forms.

components/autofill/content/renderer/password_form_conversion_utils.cc

Index: components/autofill/content/renderer/password_form_conversion_utils.cc
diff --git a/components/autofill/content/renderer/password_form_conversion_utils.cc b/components/autofill/content/renderer/password_form_conversion_utils.cc
index 44fc3230804714760d65c1d24e5e0a4becc683f2..d0e0afda9bbd07854d247fdaaaa9575f0c57acea 100644
--- a/components/autofill/content/renderer/password_form_conversion_utils.cc
+++ b/components/autofill/content/renderer/password_form_conversion_utils.cc
@@ -8,6 +8,7 @@
 
 #include "base/lazy_instance.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
 #include "base/strings/string_util.h"
 #include "components/autofill/content/renderer/form_autofill_util.h"
 #include "components/autofill/core/common/form_data_predictions.h"
@@ -239,6 +240,7 @@ void GetPasswordForm(
   password_form->username_marked_by_site = false;
   std::vector<WebInputElement> passwords;
   std::vector<base::string16> other_possible_usernames;
+  int num_text_and_password_fields = 0;
 
   WebVector<WebFormControlElement> control_elements;
   form.getFormControlElements(control_elements);
@@ -259,6 +261,12 @@ void GetPasswordForm(
         layout_sequence.push_back('P');
       else
         layout_sequence.push_back('N');
+
+      // Count the total number of text and password fields. Note that

[vabr (Chromium), 2015/05/12 10:26:02]

nit: The comment is clear and correct, but probably not needed. The name of the
variable is good enough to convey the needed meaning.

[msramek, 2015/05/12 10:50:44]

Acknowledged.

+      // in renderer, |PasswordInputType| is a subclass of |TextFieldInputType|,
+      // but for consistency with the rest of the code we use the term "text"
+      // to refer specifically to a non-password text field.
+      ++num_text_and_password_fields;
     }
 
     // If the password field is readonly, the page is likely using a virtual
@@ -368,6 +376,12 @@ void GetPasswordForm(
       }
     }
     password_form->username_value = username_value;
+  } else {

[vabr (Chromium), 2015/05/12 10:26:02]

Do you want to report this even if there are not passwords found?

[msramek, 2015/05/12 10:50:44]

Do you mean no password fields found? No. But if I understand correctly, this
method is only called for forms that we already know to contain a password
field, right?

(at least my quick testing confirms that; I didn't check for all call chains)

If you mean for forms where we have no password saved, then yes. Currently, if
you encounter a form with no username and successfully submit it, we will always
offer to save. Therefore, any such form could have resulted in saved credentials
with no username, and thus I want to get a statistic on all the forms in the
wild.

[vabr (Chromium), 2015/05/12 11:06:11]

Yes, sorry for the confusion, I meant password fields, not values.
I don't think so, at least I could not find any evidence for it in the code.

If you look just below the block you added, there is a specific function to
locate the password fields, which also handles the case when |passwords| is
empty. It might be a good idea to check for that here as well.

[msramek, 2015/05/12 11:27:03]

Interesting. I would expect none of this code to trigger on such fields, as
confirmed by my quick test. Nevertheless, you're right that 
LocateSpecificPasswords() checks this case.

I moved this block to the end of the method. LocateSpecificPasswords() only
returns true if either |password| or |new_password| has been found, so no
additional conditions are needed.

[vabr (Chromium), 2015/05/12 11:33:58]

This has the caveat that you won't get this count for password forms with 3+
password fields, where the first 3 are non-empty and equal, or all different,
etc. (see when LocateSpecificPasswords returns false).

[msramek, 2015/05/12 12:13:47]

I know. But in this case the password manager will also bail, not offering to
save the form. So for the investigation of saved empty usernames, that is ok.

+    // To get a better idea on how forms without a username field look like,
+    // report the total number of text and password fields.
+    UMA_HISTOGRAM_COUNTS_100(
+        "PasswordManager.EmptyUsernames.TextAndPasswordFieldCount",
+        num_text_and_password_fields);

[vabr (Chromium), 2015/05/12 10:26:02]

Alternatively, you can use layout_sequence.size().
But if you prefer the dedicated counter for readability, that's fine.

[msramek, 2015/05/12 10:50:44]

No, let's use layout_sequence.size(). I just really didn't notice. The advantage
here is that I'll avoid the confusion with semantics of "text field".

   }
 
   WebInputElement password;