Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/null_worker_scheduler.cc

 // Copyright 2014 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 "components/scheduler/child/null_worker_scheduler.h"
 
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/scheduler/child/null_idle_task_runner.h"
@@ skipping 11 matching lines @@
 scoped_refptr<base::SingleThreadTaskRunner>
 NullWorkerScheduler::DefaultTaskRunner() {
   return task_runner_;
 }
 
 scoped_refptr<SingleThreadIdleTaskRunner>
 NullWorkerScheduler::IdleTaskRunner() {
   return idle_task_runner_;
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+NullWorkerScheduler::ShutdownTaskRunner() {
+  return task_runner_;
+}
+
 void NullWorkerScheduler::AddTaskObserver(
     base::MessageLoop::TaskObserver* task_observer) {
   base::MessageLoop::current()->AddTaskObserver(task_observer);
 }
 
 void NullWorkerScheduler::RemoveTaskObserver(
     base::MessageLoop::TaskObserver* task_observer) {
   base::MessageLoop::current()->RemoveTaskObserver(task_observer);
 }
 
 bool NullWorkerScheduler::CanExceedIdleDeadlineIfRequired() const {
   return false;
 }
 
 bool NullWorkerScheduler::ShouldYieldForHighPriorityWork() {
   return false;
 }
 
 void NullWorkerScheduler::Init() {
 }
 
+void NullWorkerScheduler::PreShutdown() {
+}
+
 void NullWorkerScheduler::Shutdown() {
 }
 
 }  // namespace scheduler