Add a password field count histogram for empty usernames investigation.

We already have the histogram PasswordManager.EmptyUsernames.TextAndPasswordFieldCount which reports the total count of username and password fields, but to better understand it, it would help to also have a separate count of only password fields.

BUG=456728

Review URL: https://codereview.chromium.org/1149293007

Cr-Commit-Position: refs/heads/master@{#333249}

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;

tools/metrics/histograms/histograms.xml

@@ -27225,6 +27225,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.EmptyUsernames.PasswordFieldCount">
+  <owner>msramek@chromium.org</owner>
+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    The number of password fields for password forms that do not have a username
+    field. This is recorded every time such a password form is successfully
+    parsed. Note that the parsing is attempted when the form is encountered (i.e
+    when the document is loaded) and also when it is submitted.
+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.EmptyUsernames.TextAndPasswordFieldCount">
   <owner>msramek@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>