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 1775d16a1b7fb69218939aeb0f1686f9e12e7306..98984f56cc96f8f84f1a7e108e0740a0bb36e413 100644
--- a/components/ukm/ukm_service.h
+++ b/components/ukm/ukm_service.h
@@ -83,6 +83,10 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
   // the provided PrefRegistry.
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
+  // Allocates a new source id. The allocated source id is guaranteed to be
+  // unique for the duration of this browser session.
+  static int32_t NewSourceId();
+
   using AddEntryCallback = base::Callback<void(std::unique_ptr<UkmEntry>)>;
 
  protected:
@@ -90,6 +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);