Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/renderer/renderer_scheduler_impl.cc

Index: components/scheduler/renderer/renderer_scheduler_impl.cc
diff --git a/components/scheduler/renderer/renderer_scheduler_impl.cc b/components/scheduler/renderer/renderer_scheduler_impl.cc
index 4ca86ecf75ed873f8882be9a0b299203e6d2cd9b..4a3b44d681f208c1577dbee012d6883842ce3cd0 100644
--- a/components/scheduler/renderer/renderer_scheduler_impl.cc
+++ b/components/scheduler/renderer/renderer_scheduler_impl.cc
@@ -60,6 +60,10 @@ RendererSchedulerImpl::~RendererSchedulerImpl() {
       this);
 }
 
+void RendererSchedulerImpl::PreShutdown() {
+  helper_.PreShutdown();
+}
+
 void RendererSchedulerImpl::Shutdown() {
   helper_.Shutdown();
 }
@@ -92,6 +96,12 @@ RendererSchedulerImpl::TimerTaskRunner() {
   return timer_task_runner_;
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+RendererSchedulerImpl::ShutdownTaskRunner() {
+  helper_.CheckOnValidThread();
+  return helper_.ShutdownTaskRunner();
+}
+
 bool RendererSchedulerImpl::CanExceedIdleDeadlineIfRequired() const {
   return helper_.CanExceedIdleDeadlineIfRequired();
 }