Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/html_viewer/web_scheduler_impl.cc

Index: components/html_viewer/web_scheduler_impl.cc
diff --git a/components/html_viewer/web_scheduler_impl.cc b/components/html_viewer/web_scheduler_impl.cc
index 7f1ada5c1fb87506119f28da1b587086ed0ff913..f7202bfd323bd6d59b91b784d4785c7f7ee5059a 100644
--- a/components/html_viewer/web_scheduler_impl.cc
+++ b/components/html_viewer/web_scheduler_impl.cc
@@ -46,12 +46,30 @@ void WebSchedulerImpl::postTimerTask(
   scoped_ptr<blink::WebThread::Task> scoped_task(task);
   tracked_objects::Location location(web_location.functionName(),
                                      web_location.fileName(), -1, nullptr);
-  task_runner_->PostDelayedTask(
+  task_runner_->PostNonNestableDelayedTask(
       location,
       base::Bind(&WebSchedulerImpl::RunTask, base::Passed(&scoped_task)),
       base::TimeDelta::FromMilliseconds(delayMs));
 }
 
+void WebSchedulerImpl::postShutdownTask(
+    const blink::WebTraceLocation& web_location,
+    blink::WebThread::Task* task,
+    long long delayMs) {
+  scoped_ptr<blink::WebThread::Task> scoped_task(task);
+  tracked_objects::Location location(web_location.functionName(),
+                                     web_location.fileName(), -1, nullptr);
+  task_runner_->PostNonNestableDelayedTask(
+      location,
+      base::Bind(&WebSchedulerImpl::RunTask, base::Passed(&scoped_task)),
+      base::TimeDelta::FromMilliseconds(delayMs));
+}
+
+void WebSchedulerImpl::preShutdown() {
+  // TODO(alexclarke) Implement pre shutdown for mojo?
+  NOTIMPLEMENTED();
+}
+
 // static
 void WebSchedulerImpl::RunIdleTask(
     scoped_ptr<blink::WebThread::IdleTask> task) {