Convert first contentful paint logging to the new UKM client API

components/ukm/test_ukm_service.h

Index: components/ukm/test_ukm_service.h
diff --git a/components/ukm/test_ukm_service.h b/components/ukm/test_ukm_service.h
index 39ee475da893d22e0c1df1a2312ce007d58f55fe..33c81f28f2f01b653c394cf6561504c654920749 100644
--- a/components/ukm/test_ukm_service.h
+++ b/components/ukm/test_ukm_service.h
@@ -24,6 +24,9 @@ class TestUkmService : public UkmService {
   size_t sources_count() const { return sources_for_testing().size(); }
   const UkmSource* GetSource(size_t source_num) const;
 
+  size_t entries_count() const { return entries_for_testing().size(); }
+  const UkmEntry* GetEntry(size_t entry_num) const;
+
  private:
   metrics::TestMetricsServiceClient test_metrics_service_client_;