Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_source.cc

Index: components/ukm/ukm_source.cc
diff --git a/components/ukm/ukm_source.cc b/components/ukm/ukm_source.cc
index d521668bcec53e0dc327f11f481543957470c764..dc74b7d43cbb98fe89f0eb2b8a36cf64ac14a35b 100644
--- a/components/ukm/ukm_source.cc
+++ b/components/ukm/ukm_source.cc
@@ -13,14 +13,12 @@ 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