Introduce webrtc::TaskQueue and TaskQueueFactory interfaces

Bug: webrtc:10191
Change-Id: Ia2fff34cb260d904f25f7263051695f1c004a53b
Reviewed-on: https://webrtc-review.googlesource.com/c/117360
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26295}

api/task_queue/task_queue_base.cc

@@ -0,0 +1,35 @@
+/*
+ *  Copyright 2019 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 "api/task_queue/task_queue_base.h"
+
+#include "absl/base/attributes.h"
+
+namespace webrtc {
+namespace {
+
+ABSL_CONST_INIT thread_local TaskQueueBase* current = nullptr;
+
+}  // namespace
+
+TaskQueueBase* TaskQueueBase::Current() {
+  return current;
+}
+
+TaskQueueBase::CurrentTaskQueueSetter::CurrentTaskQueueSetter(
+    TaskQueueBase* task_queue)
+    : previous_(current) {
+  current = task_queue;
+}
+
+TaskQueueBase::CurrentTaskQueueSetter::~CurrentTaskQueueSetter() {
+  current = previous_;
+}
+
+}  // namespace webrtc