Introduce global task queue factory.

Bug: webrtc:10191
Change-Id: I7bdc97fd626da955b9194a9a0d8ed4f5aebddf66
Reviewed-on: https://webrtc-review.googlesource.com/c/118120
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26315}

api/task_queue/global_task_queue_factory.cc

@@ -0,0 +1,40 @@
+/*
+ *  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/global_task_queue_factory.h"
+
+#include "api/task_queue/default_task_queue_factory.h"
+#include "rtc_base/checks.h"
+
+namespace webrtc {
+namespace {
+
+TaskQueueFactory* GlobalOrDefault(TaskQueueFactory* global) {
+  static TaskQueueFactory* const factory =
+      global ? global : CreateDefaultTaskQueueFactory().release();
+  return factory;
+}
+
+}  // namespace
+
+void SetGlobalTaskQueueFactory(std::unique_ptr<TaskQueueFactory> factory) {
+  RTC_CHECK(factory) << "Can't set nullptr TaskQueueFactory";
+  // Own, but never delete the global factory.
+  TaskQueueFactory* global = factory.release();
+  RTC_CHECK(GlobalOrDefault(global) == global)
+      << "Task queue factory set after another SetFactory or after a task "
+         "queue was created";
+}
+
+TaskQueueFactory& GlobalTaskQueueFactory() {
+  return *GlobalOrDefault(nullptr);
+}
+
+}  // namespace webrtc