[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/scheduler_thread_pool.h

Index: base/task_scheduler/scheduler_thread_pool.h
diff --git a/base/task_scheduler/scheduler_thread_pool.h b/base/task_scheduler/scheduler_thread_pool.h
index 774b11137652b9f2c4efe1359c3cb2b67ffab24f..7d9b87f173316e0fb092b2c52fe9f65c0daa8b24 100644
--- a/base/task_scheduler/scheduler_thread_pool.h
+++ b/base/task_scheduler/scheduler_thread_pool.h
@@ -45,6 +45,7 @@ class BASE_EXPORT SchedulerThreadPool {
   // SchedulerThreadPool); otherwise, |task| will be added to the pending shared
   // work. |task| won't be executed before its delayed run time, if any. Returns
   // true if |task| is posted.
+  //FIXME: Remove from public interface?
   virtual bool PostTaskWithSequence(std::unique_ptr<Task> task,
                                     scoped_refptr<Sequence> sequence,
                                     SchedulerWorkerThread* worker_thread) = 0;