Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_service.h

Index: components/ukm/ukm_service.h
diff --git a/components/ukm/ukm_service.h b/components/ukm/ukm_service.h
index 2a516dfa6500ff10d693f96833af99f2bb28ac67..a8816ab58cf30a2dad6d2817cef723339e38a6d8 100644
--- a/components/ukm/ukm_service.h
+++ b/components/ukm/ukm_service.h
@@ -94,6 +94,10 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
     return sources_;
   }
 
+  const std::vector<std::unique_ptr<UkmEntry>>& entries_for_testing() const {
+    return entries_;
+  }
+
  private:
   friend UkmPageLoadMetricsObserver;
   FRIEND_TEST_ALL_PREFIXES(UkmServiceTest, AddEntryOnlyWithNonEmptyMetrics);