Add Darwin thread.h implementation.

webrtc/base/scoped_autorelease_pool.mm

-/*
- *  Copyright 2008 The WebRTC Project Authors. All rights reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#import <Foundation/Foundation.h>
-
-#import "webrtc/base/scoped_autorelease_pool.h"
-
-namespace rtc {
-
-ScopedAutoreleasePool::ScopedAutoreleasePool() {
-  pool_ = [[NSAutoreleasePool alloc] init];
-}
-
-ScopedAutoreleasePool::~ScopedAutoreleasePool() {
-  [pool_ drain];
-}
-
-}  // namespace rtc