Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_entry.h

Index: components/ukm/ukm_entry.h
diff --git a/components/ukm/ukm_entry.h b/components/ukm/ukm_entry.h
index 211c74f4c7be6ee17d90720c9deacd9d46feba14..d432bd0c1dd6f0739120a22f09b6d414d42e57c6 100644
--- a/components/ukm/ukm_entry.h
+++ b/components/ukm/ukm_entry.h
@@ -26,7 +26,9 @@ class UkmEntryBuilder;
 class UkmEntry {
  public:
   // Serializes the members of the class into the supplied proto.
-  void PopulateProto(Entry* proto_entry);
+  void PopulateProto(Entry* proto_entry) const;
+
+  int32_t source_id() const { return source_id_; }
 
   ~UkmEntry();