Deprecate two empty-username-related metrics

This CL stops logging into, and deprecates:
PasswordManager.EmptyUsernames.TextAndPasswordFieldCount and
PasswordManager.EmptyUsernames.PasswordFieldCount.

Those metrics were used for an investigation, which has concluded back
in 2016.

Bug: 852772
Change-Id: Ib15807b8384d9053d8df7e09465bbef16f9145fa
Reviewed-on: https://chromium-review.googlesource.com/1209503Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589293}

components/autofill/content/renderer/password_form_conversion_utils.cc

@@ -820,19 +820,6 @@ bool GetPasswordForm(
   }
   password_form->other_possible_usernames = std::move(other_possible_usernames);
 
-  // Report metrics.
-  if (!username_field) {
-    // To get a better idea on how password forms without a username field
-    // look like, report the total number of text and password fields.
-    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->origin = std::move(form_origin);
   password_form->signon_realm = GetSignOnRealm(password_form->origin);
   password_form->scheme = PasswordForm::SCHEME_HTML;

tools/metrics/histograms/histograms.xml

@@ -71564,6 +71564,10 @@ uploading your change for review.
 </histogram>
 
 <histogram name="PasswordManager.EmptyUsernames.PasswordFieldCount">
+  <obsolete>
+    Deprecated as of 09/2018. The investigation this metric was supporting has
+    been concluded in 2016.
+  </obsolete>
   <owner>msramek@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>
   <summary>
@@ -71575,6 +71579,10 @@ uploading your change for review.
 </histogram>
 
 <histogram name="PasswordManager.EmptyUsernames.TextAndPasswordFieldCount">
+  <obsolete>
+    Deprecated as of 09/2018. The investigation this metric was supporting has
+    been concluded in 2016.
+  </obsolete>
   <owner>msramek@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>
   <summary>