Get rid of thread_darwin file.

This file has been causing problems for the build. ObjC was required for
a few methods because autoreleasepools are necessary on new threads if
those threads will be running objc code.

This CL introduces a workaround by using ObjC runtime C APIs to create
and drain autoreleasepools, but this comes with the cost of relying on
an internal API that may break on future OS/clang releases.

Bug: webrtc:9838
Change-Id: I18e765020c20c096c9ef8d80dfa82375ecb202ff
Reviewed-on: https://webrtc-review.googlesource.com/c/105301
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25141}

rtc_base/system/cocoa_threading.h

@@ -0,0 +1,24 @@
+/*
+ *  Copyright 2018 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.
+ */
+
+#ifndef RTC_BASE_SYSTEM_COCOA_THREADING_H_
+#define RTC_BASE_SYSTEM_COCOA_THREADING_H_
+
+// If Cocoa is to be used on more than one thread, it must know that the
+// application is multithreaded.  Since it's possible to enter Cocoa code
+// from threads created by pthread_thread_create, Cocoa won't necessarily
+// be aware that the application is multithreaded.  Spawning an NSThread is
+// enough to get Cocoa to set up for multithreaded operation, so this is done
+// if necessary before pthread_thread_create spawns any threads.
+//
+// http://developer.apple.com/documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/chapter_4_section_4.html
+void InitCocoaMultiThreading();
+
+#endif  // RTC_BASE_SYSTEM_COCOA_THREADING_H_