Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/null_worker_scheduler.cc

Index: components/scheduler/child/null_worker_scheduler.cc
diff --git a/components/scheduler/child/null_worker_scheduler.cc b/components/scheduler/child/null_worker_scheduler.cc
index 4abfb0e8e7cedd69acc0351b077c8c34147036e8..6af52dcdbd7bc3a5cd1666a627e0801a5b0fe039 100644
--- a/components/scheduler/child/null_worker_scheduler.cc
+++ b/components/scheduler/child/null_worker_scheduler.cc
@@ -29,6 +29,11 @@ NullWorkerScheduler::IdleTaskRunner() {
   return idle_task_runner_;
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+NullWorkerScheduler::ShutdownTaskRunner() {
+  return task_runner_;
+}
+
 void NullWorkerScheduler::AddTaskObserver(
     base::MessageLoop::TaskObserver* task_observer) {
   base::MessageLoop::current()->AddTaskObserver(task_observer);
@@ -50,6 +55,9 @@ bool NullWorkerScheduler::ShouldYieldForHighPriorityWork() {
 void NullWorkerScheduler::Init() {
 }
 
+void NullWorkerScheduler::PreShutdown() {
+}
+
 void NullWorkerScheduler::Shutdown() {
 }