[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/task.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_H_
 #define BASE_TASK_SCHEDULER_TASK_H_
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/pending_task.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
+#include "base/task_runner.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/time/time.h"
 
 namespace base {
 namespace internal {
 
 // A task is a unit of work inside the task scheduler. Support for tracing and
 // profiling inherited from PendingTask.
 struct BASE_EXPORT Task : public PendingTask {
   // |posted_from| is the site the task was posted from. |task| is the closure
   // to run. |traits| is metadata about the task. |delay| is a delay that must
   // expire before the Task runs.
   Task(const tracked_objects::Location& posted_from,
        const Closure& task,
        const TaskTraits& traits,
        const TimeDelta& delay);
   ~Task();
 
   // The TaskTraits of this task.
   const TaskTraits traits;
 
   // The time at which the task was inserted in its sequence. For an undelayed
   // task, this happens at post time. For a delayed task, this happens some
   // time after the task's delay has expired. If the task hasn't been inserted
   // in a sequence yet, this defaults to a null TimeTicks.
   TimeTicks sequenced_time;
 
-  // A reference to the SequencedTaskRunner or SingleThreadTaskRunner that
-  // posted this task, if any. Used to set ThreadTaskRunnerHandle and/or
-  // SequencedTaskRunnerHandle while the task is running.
+  // A reference to the TaskRunner that posted this task. Used to set
+  // TaskRunnerHandle's TaskScopes while the task is running. Exactly one of
+  // these should be set per Task.
   // Note: this creates an ownership cycle
   //   Sequence -> Task -> TaskRunner -> Sequence -> ...
   // but that's okay as it's broken when the Task is popped from its Sequence
   // after being executed which means this cycle forces the TaskRunner to stick
   // around until all its tasks have been executed which is a requirement to
   // support TaskRunnerHandles.
+  scoped_refptr<TaskRunner> task_runner_ref;
   scoped_refptr<SequencedTaskRunner> sequenced_task_runner_ref;
   scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner_ref;
 
  private:
   // Disallow copies to make sure no unnecessary ref-bumps are incurred. Making
   // it move-only would be an option, but isn't necessary for now.
   DISALLOW_COPY_AND_ASSIGN(Task);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_H_