[ObjC ARC] Converts ios/chrome/browser/ui/ntp/recent_tabs:recent_tabs to ARC.

ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm

Index: ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm
diff --git a/ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm b/ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm
index 42cc6123e032dc11caa77ea6e91da45f841e1f43..a51cd79795897551fb0e3d234e30380de9082d2c 100644
--- a/ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm
+++ b/ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.mm
@@ -8,7 +8,6 @@
 #include <memory>
 
 #include "base/logging.h"
-#include "base/mac/scoped_nsobject.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/strings/utf_string_conversions.h"
@@ -16,6 +15,10 @@
 #include "components/sync_sessions/open_tabs_ui_delegate.h"
 #include "ios/chrome/browser/sync/sync_setup_service.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // Comparator function for sorting the sessions_ vector so that the most