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

third_party/libjingle_xmpp/task_runner/taskrunner.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_TASKRUNNER_H__
+#define WEBRTC_BASE_TASKRUNNER_H__
+
+#include <stdint.h>
+
+#include <vector>
+
+#include "third_party/libjingle_xmpp/task_runner/taskparent.h"
+#include "third_party/webrtc/base/checks.h"
+#include "third_party/webrtc/base/sigslot.h"
+
+namespace rtc {
+class Task;
+
+const int64_t kSecToMsec = 1000;
+const int64_t kMsecTo100ns = 10000;
+const int64_t kSecTo100ns = kSecToMsec * kMsecTo100ns;
+
+class TaskRunner : public TaskParent, public sigslot::has_slots<> {
+ public:
+  TaskRunner();
+  ~TaskRunner() override;
+
+  virtual void WakeTasks() = 0;
+
+  // Returns the current time in 100ns units.  It is used for
+  // determining timeouts.  The origin is not important, only
+  // the units and that rollover while the computer is running.
+  //
+  // On Windows, GetSystemTimeAsFileTime is the typical implementation.
+  virtual int64_t CurrentTime() = 0;
+
+  void StartTask(Task *task);
+  void RunTasks();
+  void PollTasks();
+
+  void UpdateTaskTimeout(Task* task, int64_t previous_task_timeout_time);
+
+#if RTC_DCHECK_IS_ON
+  bool is_ok_to_delete(Task* task) {
+    return task == deleting_task_;
+  }
+
+  void IncrementAbortCount() {
+    ++abort_count_;
+  }
+
+  void DecrementAbortCount() {
+    --abort_count_;
+  }
+#endif
+
+  // Returns the next absolute time when a task times out
+  // OR "0" if there is no next timeout.
+  int64_t next_task_timeout() const;
+
+ protected:
+  // The primary usage of this method is to know if
+  // a callback timer needs to be set-up or adjusted.
+  // This method will be called
+  //  * when the next_task_timeout() becomes a smaller value OR
+  //  * when next_task_timeout() has changed values and the previous
+  //    value is in the past.
+  //
+  // If the next_task_timeout moves to the future, this method will *not*
+  // get called (because it subclass should check next_task_timeout()
+  // when its timer goes off up to see if it needs to set-up a new timer).
+  //
+  // Note that this maybe called conservatively.  In that it may be
+  // called when no time change has happened.
+  virtual void OnTimeoutChange() {
+    // by default, do nothing.
+  }
+
+ private:
+  void InternalRunTasks(bool in_destructor);
+  void CheckForTimeoutChange(int64_t previous_timeout_time);
+
+  std::vector<Task *> tasks_;
+  Task *next_timeout_task_ = nullptr;
+  bool tasks_running_ = false;
+#if RTC_DCHECK_IS_ON
+  int abort_count_ = 0;
+  Task* deleting_task_ = nullptr;
+#endif
+
+  void RecalcNextTimeout(Task *exclude_task);
+};
+
+} // namespace rtc
+
+#endif  // TASK_BASE_TASKRUNNER_H__