[ObjC ARC] Converts ios/chrome/common/app_group to ARC

ios/chrome/common/app_group/app_group_metrics_mainapp.mm

 // Copyright 2015 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/common/app_group/app_group_metrics_mainapp.h"
 
 #include <stdint.h>
 
 #include "base/logging.h"
 #include "base/mac/scoped_nsobject.h"
 #include "ios/chrome/common/app_group/app_group_constants.h"
 #include "ios/chrome/common/app_group/app_group_metrics.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace app_group {
 
 namespace main_app {
 
 void ProcessPendingLogs(
     const base::mac::ScopedBlock<ProceduralBlockWithData>& callback) {
   NSFileManager* file_manager = [NSFileManager defaultManager];
   NSURL* store_url = [file_manager
       containerURLForSecurityApplicationGroupIdentifier:ApplicationGroup()];
   NSURL* log_dir_url =
@@ skipping 37 matching lines @@
 
 void DisableMetrics() {
   base::scoped_nsobject<NSUserDefaults> shared_defaults(
       [[NSUserDefaults alloc] initWithSuiteName:ApplicationGroup()]);
   [shared_defaults removeObjectForKey:@(kChromeAppClientID)];
 }
 
 }  // namespace main_app
 
 }  // namespace app_group