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..6d1e2fc25277b7c35693ec28d4c4ab72f1c458d5 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"
@@ -368,6 +369,12 @@ void GetPasswordForm(
       }
     }
     password_form->username_value = username_value;
+  } else {
+    // 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",
+        layout_sequence.size());
   }
 
   WebInputElement password;