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.h

@@ -0,0 +1,27 @@
+/*
+ *  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.
+ */
+#ifndef API_TASK_QUEUE_GLOBAL_TASK_QUEUE_FACTORY_H_
+#define API_TASK_QUEUE_GLOBAL_TASK_QUEUE_FACTORY_H_
+
+#include <memory>
+
+#include "api/task_queue/task_queue_factory.h"
+
+namespace webrtc {
+
+// May be called at most once, and before any TaskQueue is created.
+void SetGlobalTaskQueueFactory(std::unique_ptr<TaskQueueFactory> factory);
+
+// Returns TaskQueue factory. Always returns the same factory.
+TaskQueueFactory& GlobalTaskQueueFactory();
+
+}  // namespace webrtc
+
+#endif  // API_TASK_QUEUE_GLOBAL_TASK_QUEUE_FACTORY_H_