[iOS/OS X] Allow base::scoped_nsobject<> to be used when ARC is enabled.

base/mac/scoped_nsobject.mm

+// Copyright 2016 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.
+
+#import "base/mac/scoped_nsobject.h"
+
+namespace base {
+namespace internal {
+
+id ScopedNSProtocolTraitsRetain(id obj) {
+  return [obj retain];
+}
+
+id ScopedNSProtocolTraitsAutoRelease(id obj) {
+  return [obj autorelease];
+}
+
+void ScopedNSProtocolTraitsRelease(id obj) {
+  return [obj release];
+}
+
+}  // namespace internal
+}  // namespace base