Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/worker_scheduler_impl.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_WORKER_SCHEDULER_IMPL_H_
 #define COMPONENTS_SCHEDULER_CHILD_WORKER_SCHEDULER_IMPL_H_
 
 #include "components/scheduler/child/scheduler_helper.h"
 #include "components/scheduler/child/worker_scheduler.h"
 #include "components/scheduler/scheduler_export.h"
@@ skipping 12 matching lines @@
     : public WorkerScheduler,
       public SchedulerHelper::SchedulerHelperDelegate {
  public:
   explicit WorkerSchedulerImpl(
       scoped_refptr<NestableSingleThreadTaskRunner> main_task_runner);
   ~WorkerSchedulerImpl() override;
 
   // WorkerScheduler implementation:
   scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() override;
   scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> ShutdownTaskRunner() override;
   bool CanExceedIdleDeadlineIfRequired() const override;
   bool ShouldYieldForHighPriorityWork() override;
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer) override;
   void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) override;
   void Init() override;
+  void PreShutdown() override;
   void Shutdown() override;
 
   SchedulerHelper* GetSchedulerHelperForTesting();
   base::TimeTicks CurrentIdleTaskDeadlineForTesting() const;
 
  protected:
   // SchedulerHelperDelegate implementation:
   bool CanEnterLongIdlePeriod(
       base::TimeTicks now,
       base::TimeDelta* next_long_idle_period_delay_out) override;
   void IsNotQuiescent() override {}
 
  private:
   void MaybeStartLongIdlePeriod();
 
   SchedulerHelper helper_;
   bool initialized_;
 
   DISALLOW_COPY_AND_ASSIGN(WorkerSchedulerImpl);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_WORKER_SCHEDULER_IMPL_H_