[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/scheduler_thread_pool_impl.cc

Index: base/task_scheduler/scheduler_thread_pool_impl.cc
diff --git a/base/task_scheduler/scheduler_thread_pool_impl.cc b/base/task_scheduler/scheduler_thread_pool_impl.cc
index 44667e99d23cb9e189f55b1819bfb859a315d295..9863e8497f9e5e7be842ffe8fc5e3602a6473cf4 100644
--- a/base/task_scheduler/scheduler_thread_pool_impl.cc
+++ b/base/task_scheduler/scheduler_thread_pool_impl.cc
@@ -49,10 +49,12 @@ class SchedulerParallelTaskRunner : public TaskRunner {
   bool PostDelayedTask(const tracked_objects::Location& from_here,
                        const Closure& closure,
                        TimeDelta delay) override {
+    std::unique_ptr<Task> task(new Task(from_here, closure, traits_, delay));
+    task->task_runner_ref = this;
+
     // Post the task as part of a one-off single-task Sequence.
     return thread_pool_->PostTaskWithSequence(
-        WrapUnique(new Task(from_here, closure, traits_, delay)),
-        make_scoped_refptr(new Sequence), nullptr);
+        std::move(task), make_scoped_refptr(new Sequence), nullptr);
   }
 
   bool RunsTasksOnCurrentThread() const override {