Reland of New task queueing primitive for async tasks: TaskQueue.

webrtc/base/task_queue_posix.h

 /*
  *  Copyright 2016 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.
  */
 
-#include "webrtc/base/logging.h"
+#ifndef WEBRTC_BASE_TASK_QUEUE_POSIX_H_
+#define WEBRTC_BASE_TASK_QUEUE_POSIX_H_
 
-#import <Foundation/Foundation.h>
-
+#include <pthread.h>
 
 namespace rtc {
-std::string DescriptionFromOSStatus(OSStatus err) {
-  NSError* error =
-      [NSError errorWithDomain:NSOSStatusErrorDomain code:err userInfo:nil];
-  return error.description.UTF8String;
-}
+
+class TaskQueue;
+
+namespace internal {
+
+class AutoSetCurrentQueuePtr {
+ public:
+  explicit AutoSetCurrentQueuePtr(TaskQueue* q);
+  ~AutoSetCurrentQueuePtr();
+
+ private:
+  TaskQueue* const prev_;
+};
+
+pthread_key_t GetQueuePtrTls();
+
+}  // namespace internal
 }  // namespace rtc
+
+#endif  // WEBRTC_BASE_TASK_QUEUE_POSIX_H_