Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_source.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/ukm/ukm_source.h"
 
 #include "base/hash.h"
 #include "components/metrics/proto/ukm/source.pb.h"
 
 namespace ukm {
 
 UkmSource::UkmSource() = default;
 
 UkmSource::~UkmSource() = default;
 
-void UkmSource::PopulateProto(Source* proto_source) {
+void UkmSource::PopulateProto(Source* proto_source) const {
   DCHECK(!proto_source->has_id());
   DCHECK(!proto_source->has_url());
 
   proto_source->set_id(id_);
-  proto_source->set_url(committed_url_.spec());
-  proto_source->set_first_contentful_paint_msec(
-      first_contentful_paint_.InMilliseconds());
+  proto_source->set_url(url_.spec());
 }
 
 }  // namespace ukm