Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/renderer/null_renderer_scheduler.h

Index: components/scheduler/renderer/null_renderer_scheduler.h
diff --git a/components/scheduler/renderer/null_renderer_scheduler.h b/components/scheduler/renderer/null_renderer_scheduler.h
index a346665c530e7659bb7ece61560930569de56bc9..51e317741671ff9f526f2d994877d1d351271171 100644
--- a/components/scheduler/renderer/null_renderer_scheduler.h
+++ b/components/scheduler/renderer/null_renderer_scheduler.h
@@ -19,6 +19,7 @@ class NullRendererScheduler : public RendererScheduler {
   scoped_refptr<base::SingleThreadTaskRunner> LoadingTaskRunner() override;
   scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner() override;
   scoped_refptr<base::SingleThreadTaskRunner> TimerTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> ShutdownTaskRunner() override;
 
   void WillBeginFrame(const cc::BeginFrameArgs& args) override;
   void BeginFrameNotExpectedSoon() override;
@@ -34,6 +35,7 @@ class NullRendererScheduler : public RendererScheduler {
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer) override;
   void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) override;
+  void PreShutdown() override;
   void Shutdown() override;
   void SuspendTimerQueue() override;
   void ResumeTimerQueue() override;