[TaskScheduler] Support TaskRunnerHandles from parallel tasks.

base/task_scheduler/task_scheduler_impl.cc

 // 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.
 
 #include "base/task_scheduler/task_scheduler_impl.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 17 matching lines @@
 TaskSchedulerImpl::~TaskSchedulerImpl() {
 #if DCHECK_IS_ON()
   DCHECK(join_for_testing_returned_.IsSignaled());
 #endif
 }
 
 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(
     const TaskTraits& traits,
     ExecutionMode execution_mode) {
   return GetThreadPoolForTraits(traits)->CreateTaskRunnerWithTraits(
       traits, execution_mode);
 }
 
 void TaskSchedulerImpl::Shutdown() {
@@ skipping 100 matching lines @@
   GetThreadPoolForTraits(traits)->ReEnqueueSequence(std::move(sequence),
                                                     sort_key);
 }
 
 void TaskSchedulerImpl::OnDelayedRunTimeUpdated() {
   service_thread_->WakeUp();
 }
 
 }  // namespace internal
 }  // namespace base