Add UMA histograms for tracking empty usernames.

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 46f71a99d38dc07a9c9f82eee752085e781747b7..c9cfbe991f707e8eb8c3300f3e850ad223b399da 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23260,6 +23260,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.NumEmptyUsernames">

[Ilya Sherman, 2015/01/29 20:31:09]

nit: Suggested name: "PasswordManager.EmptyUsernames.CountInDatabase".

[vasilii, 2015/01/30 12:35:41]

Done.

+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    Number of password forms with empty username in the Password Store.

[vabr (Chromium), 2015/01/29 15:40:23]

Could you please insert a <!--comment--> with a TODO pointing to the bug for
adding UMA reports to PasswordStoreX?

[Ilya Sherman, 2015/01/29 20:31:09]

Please document when this metric is recorded -- at startup?  On access to the
DB?  Other?

[vasilii, 2015/01/30 12:35:41]

I didn't find similar comments in the xml. I don't think it's a problem for this
particular diagram, for others it may be worse.

[vasilii, 2015/01/30 12:35:41]

Done.

+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.NumPasswordsDeletedByBulkDelete">
   <owner>dubroy@chromium.org</owner>
   <owner>vabr@chromium.org</owner>
@@ -23296,6 +23303,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.OfferSaveEmptyUsername">

[Ilya Sherman, 2015/01/29 20:31:09]

Suggested name: "PasswordManager.EmptyUsernames.OfferedToSave".  The main part
of these suggestions is moving "EmptyUsernames" to the front of the name, so
that the two related stats are grouped together hierarchically and
alphabetically.

[Ilya Sherman, 2015/01/29 20:31:09]

Please associate an enum that's specifically relevant to this histogram.

[vasilii, 2015/01/30 12:35:41]

Done.

[vasilii, 2015/01/30 12:35:42]

Done.

+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    Indicates if the user is proposed to save an empty username.

[Ilya Sherman, 2015/01/29 20:31:09]

Please document when this metric is recorded.  So, something like: "When
offering to save a password, whether the username is empty." or "How frequently
users are prompted to save empty usernames.  Recorded each time Chrome offers to
save a password." or something along those lines.

[vasilii, 2015/01/30 12:35:42]

Done.

+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.OsPasswordStatus"
     enum="PasswordManagerOsPasswordStatus">
   <owner>dubroy@chromium.org</owner>