[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/task.h

Index: base/task_scheduler/task.h
diff --git a/base/task_scheduler/task.h b/base/task_scheduler/task.h
index 2b53c690fddffc752ce63ec98b900a0b34f06674..2a448897e3b70fb44aa4e6d2cb9118f18d14cc51 100644
--- a/base/task_scheduler/task.h
+++ b/base/task_scheduler/task.h
@@ -13,6 +13,7 @@
 #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"
 
@@ -40,15 +41,16 @@ struct BASE_EXPORT Task : public PendingTask {
   // 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;