Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/worker_scheduler_impl.cc

Index: components/scheduler/child/worker_scheduler_impl.cc
diff --git a/components/scheduler/child/worker_scheduler_impl.cc b/components/scheduler/child/worker_scheduler_impl.cc
index cfd444e93839c25ca2f4d5cb355ed245a8c4cdfc..726ea1529b2db9ffa35b83e9bb13c4f4c78903a0 100644
--- a/components/scheduler/child/worker_scheduler_impl.cc
+++ b/components/scheduler/child/worker_scheduler_impl.cc
@@ -48,6 +48,12 @@ WorkerSchedulerImpl::IdleTaskRunner() {
   return helper_.IdleTaskRunner();
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+WorkerSchedulerImpl::ShutdownTaskRunner() {
+  DCHECK(initialized_);
+  return helper_.ShutdownTaskRunner();
+}
+
 bool WorkerSchedulerImpl::CanExceedIdleDeadlineIfRequired() const {
   DCHECK(initialized_);
   return helper_.CanExceedIdleDeadlineIfRequired();
@@ -70,6 +76,11 @@ void WorkerSchedulerImpl::RemoveTaskObserver(
   helper_.RemoveTaskObserver(task_observer);
 }
 
+void WorkerSchedulerImpl::PreShutdown() {
+  DCHECK(initialized_);
+  helper_.PreShutdown();
+}
+
 void WorkerSchedulerImpl::Shutdown() {
   DCHECK(initialized_);
   helper_.Shutdown();