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

base/mac/scoped_block.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_block.h"
+

[Nico, 2016/04/04 17:53:44]

Is the idea that we'll have this file around in non-arc mode in perpetuity? If
not, when would we remove it?

+namespace base {
+namespace mac {
+namespace internal {
+
+id ScopedBlockTraitsRetain(id block) {
+  return Block_copy(block);
+}
+void ScopedBlockTraitsRelease(id block) {

[Mark Mentovai, 2016/04/05 00:09:15]

nit: Blank line between functions.

+  Block_release(block);
+}
+
+}  // namespace internal
+}  // namespace mac
+}  // namespace base