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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/ui/ntp/recent_tabs/synced_sessions.h"
 
 #include <functional>
 #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"
 #include "components/sync/driver/sync_service.h"
 #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
 // recently used sessions are at the beginning.
 bool SortSessionsByTime(
     std::unique_ptr<const synced_sessions::DistantSession>& a,
     std::unique_ptr<const synced_sessions::DistantSession>& b) {
   return a->modified_time > b->modified_time;
 }
 
@@ skipping 138 matching lines @@
   DCHECK_LE(index, GetSessionCount());
   sessions_.erase(sessions_.begin() + index);
 }
 
 void SyncedSessions::AddDistantSessionForTest(
     std::unique_ptr<const DistantSession> distant_session) {
   sessions_.push_back(std::move(distant_session));
 }
 
 }  // synced_sessions namespace