Basic UkmService implementation

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm

Index: ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm
diff --git a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm
index 43fee09cca472b15b7f37dd14a2cf8a56941fd5d..fd2314ac0d5af7b33335ba33921200bd56800650 100644
--- a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm
+++ b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm
@@ -41,6 +41,7 @@
 #include "components/signin/core/browser/signin_status_metrics_provider.h"
 #include "components/sync/device_info/device_count_metrics_provider.h"
 #include "components/translate/core/browser/translate_ranker_metrics_provider.h"
+#include "components/ukm/ukm_service.h"
 #include "components/variations/variations_associated_data.h"
 #include "components/version_info/version_info.h"
 #include "ios/chrome/browser/application_context.h"
@@ -91,12 +92,17 @@ void IOSChromeMetricsServiceClient::RegisterPrefs(
   metrics::MetricsService::RegisterPrefs(registry);
   metrics::StabilityMetricsHelper::RegisterPrefs(registry);
   metrics::RegisterMetricsReportingStatePrefs(registry);
+  ukm::UkmService::RegisterPrefs(registry);
 }
 
 metrics::MetricsService* IOSChromeMetricsServiceClient::GetMetricsService() {
   return metrics_service_.get();
 }
 
+metrics::MetricsService* IOSChromeMetricsServiceClient::GetMetricsService() {
+  return ukm_service_.get();
+}
+
 void IOSChromeMetricsServiceClient::SetMetricsClientId(
     const std::string& client_id) {
   crash_keys::SetMetricsClientIdFromGUID(client_id);
@@ -182,8 +188,12 @@ void IOSChromeMetricsServiceClient::WebStateDidStopLoading(
 }
 
 void IOSChromeMetricsServiceClient::Initialize() {
+  PrefService* local_state = GetApplicationContext()->GetLocalState();
   metrics_service_ = base::MakeUnique<metrics::MetricsService>(
-      metrics_state_manager_, this, GetApplicationContext()->GetLocalState());
+      metrics_state_manager_, this, local_state);
+
+  if (base::FeatureList::IsEnabled(kUkmFeature))
+    ukm_service_.reset(new ukm::UkmService(local_state, this));
 
   // Register metrics providers.
   metrics_service_->RegisterMetricsProvider(