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}
diff --git a/api/task_queue/task_queue.cc b/api/task_queue/task_queue.cc
new file mode 100644
index 0000000..4a33d48
--- /dev/null
+++ b/api/task_queue/task_queue.cc
@@ -0,0 +1,63 @@
+/*
+ *  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 "rtc_base/task_queue.h"
+
+#include "api/task_queue/global_task_queue_factory.h"
+#include "api/task_queue/task_queue_base.h"
+
+namespace rtc {
+
+TaskQueue::TaskQueue(const char* queue_name, Priority priority)
+    // For backward compatibility impl_ need to be scoped_refptr<Impl>,
+    // But this implementation treat impl_ as
+    // std::unique_ptr<webrtc::TaskQueueBase, webrtc::TaskQueueDeleter> abusing
+    // fact that both classes are wrappers around raw pointer.
+    : impl_(webrtc::GlobalTaskQueueFactory()
+                .CreateTaskQueue(queue_name, priority)
+                .release()) {
+  impl_->task_queue_ = this;
+}
+
+TaskQueue::~TaskQueue() {
+  // TODO(danilchap): change impl_ to webrtc::TaskQueueBase* when dependenent
+  // projects stop using link-injection to override task queue and thus do not
+  // rely on exact TaskQueue layout.
+  // There might running task that tries to rescheduler itself to the TaskQueue
+  // and not yet away TaskQueue destructor is called.
+  // Calling back to TaskQueue::PostTask need impl_ pointer still be valid, so
+  // Start the destruction first, ...
+  impl_->Delete();
+  // release the pointer later.
+  const_cast<rtc::scoped_refptr<Impl>&>(impl_).release();
+}
+
+// static
+TaskQueue* TaskQueue::Current() {
+  webrtc::TaskQueueBase* impl = webrtc::TaskQueueBase::Current();
+  if (impl == nullptr) {
+    return nullptr;
+  }
+  return impl->task_queue_;
+}
+
+bool TaskQueue::IsCurrent() const {
+  return Current() == this;
+}
+
+void TaskQueue::PostTask(std::unique_ptr<QueuedTask> task) {
+  return impl_->PostTask(std::move(task));
+}
+
+void TaskQueue::PostDelayedTask(std::unique_ptr<QueuedTask> task,
+                                uint32_t milliseconds) {
+  return impl_->PostDelayedTask(std::move(task), milliseconds);
+}
+
+}  // namespace rtc