[Sync] Hook UserEvents into sync.

components/sync/user_events/user_event_service.cc

Index: components/sync/user_events/user_event_service.cc
diff --git a/components/sync/user_events/user_event_service.cc b/components/sync/user_events/user_event_service.cc
index ac403790078bba75542117c3bd577af1205ebf6d..328808b6dcbfd6d8fa9ed6eaf4a231e102f2633b 100644
--- a/components/sync/user_events/user_event_service.cc
+++ b/components/sync/user_events/user_event_service.cc
@@ -7,8 +7,10 @@
 #include <utility>
 
 #include "base/bind.h"
+#include "base/feature_list.h"
 #include "base/memory/ptr_util.h"
 #include "base/rand_util.h"
+#include "components/sync/driver/sync_driver_switches.h"
 #include "components/sync/driver/sync_service.h"
 #include "components/sync/model/model_type_sync_bridge.h"
 #include "components/sync/protocol/sync.pb.h"
@@ -51,7 +53,8 @@ bool UserEventService::CanRecordEvent(const UserEventSpecifics& specifics) {
   // We only record events if the user is syncing history and has not enabled
   // a custom passphrase. The type HISTORY_DELETE_DIRECTIVES is enabled in and
   // only in this exact scenario.
-  return sync_service_ != nullptr && sync_service_->IsEngineInitialized() &&
+  return base::FeatureList::IsEnabled(switches::kSyncUserEvents) &&
+         sync_service_ != nullptr && sync_service_->IsEngineInitialized() &&
          sync_service_->GetPreferredDataTypes().Has(HISTORY_DELETE_DIRECTIVES);
 }