Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/null_worker_scheduler.h

 // Copyright 2015 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.
 
 #ifndef COMPONENTS_SCHEDULER_CHILD_NULL_WORKER_SCHEDULER_H_
 #define COMPONENTS_SCHEDULER_CHILD_NULL_WORKER_SCHEDULER_H_
 
 #include "components/scheduler/child/worker_scheduler.h"
 
 namespace scheduler {
 
 class NullWorkerScheduler : public WorkerScheduler {
  public:
   NullWorkerScheduler();
   ~NullWorkerScheduler() override;
 
   scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() override;
   scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> ShutdownTaskRunner() override;
 
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer) override;
   void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) override;
   bool CanExceedIdleDeadlineIfRequired() const override;
   bool ShouldYieldForHighPriorityWork() override;
   void Init() override;
+  void PreShutdown() override;
   void Shutdown() override;
 
  private:
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   scoped_refptr<SingleThreadIdleTaskRunner> idle_task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(NullWorkerScheduler);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_NULL_WORKER_SCHEDULER_H_