Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/web_scheduler_impl.h

Index: components/scheduler/child/web_scheduler_impl.h
diff --git a/components/scheduler/child/web_scheduler_impl.h b/components/scheduler/child/web_scheduler_impl.h
index 0d1b48474849f2a70b9c7805c50bc3de9cfdd979..f114269d4750298cfaa68b787d95bdc70447c4f3 100644
--- a/components/scheduler/child/web_scheduler_impl.h
+++ b/components/scheduler/child/web_scheduler_impl.h
@@ -27,12 +27,14 @@ class SCHEDULER_EXPORT WebSchedulerImpl : public blink::WebScheduler {
       ChildScheduler* child_scheduler,
       scoped_refptr<SingleThreadIdleTaskRunner> idle_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> loading_task_runner,
-      scoped_refptr<base::SingleThreadTaskRunner> timer_task_runner);
+      scoped_refptr<base::SingleThreadTaskRunner> timer_task_runner,
+      scoped_refptr<base::SingleThreadTaskRunner> shutdown_task_runner);
   ~WebSchedulerImpl() override;
 
   // blink::WebScheduler implementation:
   virtual bool shouldYieldForHighPriorityWork();
   virtual bool canExceedIdleDeadlineIfRequired();
+  virtual void preShutdown();
   virtual void postIdleTask(const blink::WebTraceLocation& location,
                             blink::WebThread::IdleTask* task);
   virtual void postNonNestableIdleTask(const blink::WebTraceLocation& location,
@@ -44,6 +46,9 @@ class SCHEDULER_EXPORT WebSchedulerImpl : public blink::WebScheduler {
   virtual void postTimerTask(const blink::WebTraceLocation& location,
                              blink::WebThread::Task* task,
                              long long delayMs);
+  virtual void postShutdownTask(const blink::WebTraceLocation& location,
+                                blink::WebThread::Task* task,
+                                long long delayMs);
 
  private:
   static void runIdleTask(scoped_ptr<blink::WebThread::IdleTask> task,
@@ -54,6 +59,7 @@ class SCHEDULER_EXPORT WebSchedulerImpl : public blink::WebScheduler {
   scoped_refptr<SingleThreadIdleTaskRunner> idle_task_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> loading_task_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> timer_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> shutdown_task_runner_;
 };
 
 }  // namespace scheduler