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

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index ab123224f2860a2b8f78e77c1057028c1e7c0773..dd6ea52693fc9391139a71817d6111468514f64f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25700,6 +25700,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.EmptyUsernames.TextAndPasswordFieldCount">
+  <owner>msramek@chromium.org</owner>
+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    The total number of text and password fields for password forms that do not
+    have a username field. This is recorded every time such a password form is
+    successfully parsed.

[Ilya Sherman, 2015/05/13 17:30:40]

Please clarify when such forms are parsed.  Is it once per page load?

[msramek, 2015/05/15 10:50:32]

Done.

It is done on page load and also on form submission. As was noted in the
previous discussion with Václav, this can create a bias towards submitted forms,
but this is not worth solving right now, as there are other biases already due
to how UMA works. We won't be able to get the exact ratio of N-field forms, but
we will at least see if there are a lot of forms for particular N.

+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.EmptyUsernames.WithoutCorrespondingNonempty">
   <owner>msramek@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>