Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/renderer/null_renderer_scheduler.cc

Index: components/scheduler/renderer/null_renderer_scheduler.cc
diff --git a/components/scheduler/renderer/null_renderer_scheduler.cc b/components/scheduler/renderer/null_renderer_scheduler.cc
index 22736b6ca38ab9c3a5ea18d8e18a544ec20a19c6..a301268a84ca0939c5a2490ec5d590e15748cd28 100644
--- a/components/scheduler/renderer/null_renderer_scheduler.cc
+++ b/components/scheduler/renderer/null_renderer_scheduler.cc
@@ -44,6 +44,11 @@ NullRendererScheduler::TimerTaskRunner() {
   return task_runner_;
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+NullRendererScheduler::ShutdownTaskRunner() {
+  return task_runner_;
+}
+
 void NullRendererScheduler::WillBeginFrame(const cc::BeginFrameArgs& args) {
 }
 
@@ -88,6 +93,9 @@ void NullRendererScheduler::RemoveTaskObserver(
   base::MessageLoop::current()->RemoveTaskObserver(task_observer);
 }
 
+void NullRendererScheduler::PreShutdown() {
+}
+
 void NullRendererScheduler::Shutdown() {
 }