Add a copy of webrtc's TaskRunner abstraction, which is going to be deleted in webrtc.

third_party/libjingle_xmpp/task_runner/taskparent.h

+/*
+ *  Copyright 2004 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 WEBRTC_BASE_TASKPARENT_H__
+#define WEBRTC_BASE_TASKPARENT_H__
+
+#include <memory>
+#include <set>
+
+#include "third_party/webrtc/base/checks.h"
+#include "third_party/webrtc/base/constructormagic.h"
+
+namespace rtc {
+
+class Task;
+class TaskRunner;
+
+class TaskParent {
+ public:
+  TaskParent(Task *derived_instance, TaskParent *parent);
+  explicit TaskParent(TaskRunner *derived_instance);
+  virtual ~TaskParent();
+
+  TaskParent *GetParent() { return parent_; }
+  TaskRunner *GetRunner() { return runner_; }
+
+  bool AllChildrenDone();
+  bool AnyChildError();
+#if RTC_DCHECK_IS_ON
+  bool IsChildTask(Task *task);
+#endif
+
+ protected:
+  void OnStopped(Task *task);
+  void AbortAllChildren();
+  TaskParent *parent() {
+    return parent_;
+  }
+
+ private:
+  void Initialize();
+  void OnChildStopped(Task *child);
+  void AddChild(Task *child);
+
+  TaskParent *parent_;
+  TaskRunner *runner_;
+  bool child_error_;
+  typedef std::set<Task *> ChildSet;
+  std::unique_ptr<ChildSet> children_;
+  RTC_DISALLOW_COPY_AND_ASSIGN(TaskParent);
+};
+
+
+} // namespace rtc
+
+#endif  // WEBRTC_BASE_TASKPARENT_H__