[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/task_scheduler_impl.cc

Index: base/task_scheduler/task_scheduler_impl.cc
diff --git a/base/task_scheduler/task_scheduler_impl.cc b/base/task_scheduler/task_scheduler_impl.cc
index 592839c74b09a3b9a12e8aed62d623e8158386a7..a96fde3e38cb4d3825d7efe9a357f0e4db00869d 100644
--- a/base/task_scheduler/task_scheduler_impl.cc
+++ b/base/task_scheduler/task_scheduler_impl.cc
@@ -35,10 +35,11 @@ void TaskSchedulerImpl::PostTaskWithTraits(
     const tracked_objects::Location& from_here,
     const TaskTraits& traits,
     const Closure& task) {
-  // Post |task| as part of a one-off single-task Sequence.
-  GetThreadPoolForTraits(traits)->PostTaskWithSequence(
-      WrapUnique(new Task(from_here, task, traits, TimeDelta())),
-      make_scoped_refptr(new Sequence), nullptr);
+  // Although this is a one-off task, still need to create a TaskRunner for it
+  // to support TaskRunnerHandles in its context.
+  GetThreadPoolForTraits(traits)
+      ->CreateTaskRunnerWithTraits(traits, ExecutionMode::PARALLEL)
+      ->PostTask(from_here, task);
 }
 
 scoped_refptr<TaskRunner> TaskSchedulerImpl::CreateTaskRunnerWithTraits(