Convert first contentful paint logging to the new UKM client API

components/ukm/test_ukm_service.cc

Index: components/ukm/test_ukm_service.cc
diff --git a/components/ukm/test_ukm_service.cc b/components/ukm/test_ukm_service.cc
index d2f4c12538037c278fc14b5cee676cad0f7f65f9..b49c3cb61e7fb7239d1ed38f77eaa513ef56366c 100644
--- a/components/ukm/test_ukm_service.cc
+++ b/components/ukm/test_ukm_service.cc
@@ -28,4 +28,8 @@ const UkmSource* TestUkmService::GetSource(size_t source_num) const {
   return sources_for_testing()[source_num].get();
 }
 
+const UkmEntry* TestUkmService::GetEntry(size_t entry_num) const {
+  return entries_for_testing()[entry_num].get();
+}
+
 }  // namespace ukm