Add a password field count histogram for empty usernames investigation.

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 e27e482ac4b46aad2da8990eb99fa7e163402eda..f4dfcdf53f6f9e99323f0aa14430391c864485f7 100644
--- a/components/autofill/content/renderer/password_form_conversion_utils.cc
+++ b/components/autofill/content/renderer/password_form_conversion_utils.cc
@@ -427,6 +427,10 @@ void GetPasswordForm(
     UMA_HISTOGRAM_COUNTS_100(
         "PasswordManager.EmptyUsernames.TextAndPasswordFieldCount",
         layout_sequence.size());
+    // For comparison, also report the number of password fields.
+    UMA_HISTOGRAM_COUNTS_100(
+        "PasswordManager.EmptyUsernames.PasswordFieldCount",
+        std::count(layout_sequence.begin(), layout_sequence.end(), 'P'));
   }
 
   password_form->scheme = PasswordForm::SCHEME_HTML;