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 60f301efb0e27b52649771cf18b8f798fddccf53..3c57352662704f011a52ee3a143a3e700d678b39 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23249,6 +23249,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.EmptyUsernames.CountInDatabase">
+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    Number of password forms with empty username in the Login Database. This is
+    recorded on startup.
+  </summary>
+</histogram>
+
+<histogram name="PasswordManager.EmptyUsernames.OfferedToSave"
+    enum="PasswordManagerShowEmptyUsername">
+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    When offering to save a password, whether the username is empty.
+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.Enabled" enum="BooleanEnabled">
   <owner>dubroy@chromium.org</owner>
   <owner>vabr@chromium.org</owner>
@@ -53917,6 +53933,11 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="2" label="Match"/>
 </enum>
 
+<enum name="PasswordManagerShowEmptyUsername" type="int">
+  <int value="0" label="Non-empty username"/>
+  <int value="1" label="Empty username"/>
+</enum>
+
 <enum name="PasswordManagerSyncingAccountState" type="int">
   <summary>
     The value is a combination of the current sync state and if the user has