Connect global task queue factory and rtc::TaskQueue

This cl allows to overwrite TaskQueue implementation
by depending on and setting the global task queue factory.

Bug: webrtc:10191
Change-Id: I69ceb139d00078d3be90eeb4e240758b88829c20
Reviewed-on: https://webrtc-review.googlesource.com/c/118060
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26345}

api/task_queue/task_queue_impl.h

@@ -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.
+ */
+
+#ifndef API_TASK_QUEUE_TASK_QUEUE_IMPL_H_
+#define API_TASK_QUEUE_TASK_QUEUE_IMPL_H_
+
+#include <memory>
+
+#include "api/task_queue/queued_task.h"
+#include "rtc_base/task_queue.h"
+
+// TODO(danilchap): Remove Impl and dependency on rtc::TaskQueue when custom
+// implementations switch to use global factories that creates TaskQueue
+// instead of using link-time injection.
+class rtc::TaskQueue::Impl {
+ public:
+  virtual void Delete() = 0;
+  virtual void PostTask(std::unique_ptr<QueuedTask> task) = 0;
+  virtual void PostDelayedTask(std::unique_ptr<QueuedTask> task,
+                               uint32_t milliseconds) = 0;
+
+  void AddRef();
+  void Release();
+
+ protected:
+  virtual ~Impl() = default;
+
+ private:
+  friend class rtc::TaskQueue;
+  rtc::TaskQueue* task_queue_ = nullptr;
+};
+
+#endif  // API_TASK_QUEUE_TASK_QUEUE_IMPL_H_