[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/task_tracker.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 #define BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 
 #include <memory>
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/macros.h"
+#include "base/memory/ref_counted.h"
 #include "base/metrics/histogram_base.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/task_scheduler/scheduler_lock.h"
 #include "base/task_scheduler/task.h"
 #include "base/task_scheduler/task_traits.h"
 
 namespace base {
 namespace internal {
 
 // All tasks go through the scheduler's TaskTracker when they are posted and
@@ skipping 62 matching lines @@
   // True once Shutdown() has returned. No new task can be scheduled after this.
   bool shutdown_completed_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTracker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_