Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

mojo/services/html_viewer/web_scheduler_impl.h

Index: mojo/services/html_viewer/web_scheduler_impl.h
diff --git a/mojo/services/html_viewer/web_scheduler_impl.h b/mojo/services/html_viewer/web_scheduler_impl.h
index 9d1563935cbf87ada582206657bfaa331a326396..a2b5b964164cf4b8014671dc58a15ccf96957142 100644
--- a/mojo/services/html_viewer/web_scheduler_impl.h
+++ b/mojo/services/html_viewer/web_scheduler_impl.h
@@ -26,6 +26,10 @@ class 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);
+  virtual void preShutdown();
 
   static void RunIdleTask(scoped_ptr<blink::WebThread::IdleTask> task);
   static void RunTask(scoped_ptr<blink::WebThread::Task> task);