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..b737f1a3c26759ccfbc182e55b878a14e24377b9 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_macros.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"
@@ -404,6 +405,14 @@ void GetPasswordForm(
       password_form->new_password_marked_by_site = true;
   }
 
+  if (username_element.isNull()) {
+    // To get a better idea on how password forms without a username field
+    // look like, report the total number of text and password fields.
+    UMA_HISTOGRAM_COUNTS_100(
+        "PasswordManager.EmptyUsernames.TextAndPasswordFieldCount",
+        layout_sequence.size());
+  }
+
   password_form->scheme = PasswordForm::SCHEME_HTML;
   password_form->ssl_valid = false;
   password_form->preferred = false;