Add Darwin thread.h implementation.

webrtc/base/maccocoathreadhelper.h

-/*
- *  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.
- */
-
-// Helper function for using Cocoa with Posix threads. This header should be
-// included from C/C++ files that want to use some Cocoa functionality without
-// using the .mm extension (mostly for files that are compiled on multiple
-// platforms).
-
-#ifndef WEBRTC_BASE_MACCOCOATHREADHELPER_H__
-#define WEBRTC_BASE_MACCOCOATHREADHELPER_H__
-
-namespace rtc {
-
-// Cocoa must be "put into multithreading mode" before Cocoa functionality can
-// be used on POSIX threads. This function does that.
-void InitCocoaMultiThreading();
-
-}  // namespace rtc
-
-#endif  // WEBRTC_BASE_MACCOCOATHREADHELPER_H__