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

webrtc/base/task_queue_posix.cc

+/*
+ *  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/task_queue_posix.h"
+
+#include "webrtc/base/checks.h"
+#include "webrtc/base/task_queue.h"
+
+namespace rtc {
+namespace internal {
+pthread_key_t g_queue_ptr_tls = 0;
+
+void InitializeTls() {
+  RTC_CHECK(pthread_key_create(&g_queue_ptr_tls, nullptr) == 0);
+}
+
+pthread_key_t GetQueuePtrTls() {
+  static pthread_once_t init_once = PTHREAD_ONCE_INIT;
+  RTC_CHECK(pthread_once(&init_once, &InitializeTls) == 0);
+  return g_queue_ptr_tls;
+}
+
+AutoSetCurrentQueuePtr::AutoSetCurrentQueuePtr(TaskQueue* q)
+    : prev_(TaskQueue::Current()) {
+  pthread_setspecific(GetQueuePtrTls(), q);
+}
+
+AutoSetCurrentQueuePtr::~AutoSetCurrentQueuePtr() {
+  pthread_setspecific(GetQueuePtrTls(), prev_);
+}
+
+}  // namespace internal
+}  // namespace rtc