Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/web_scheduler_impl.cc

Index: components/scheduler/child/web_scheduler_impl.cc
diff --git a/components/scheduler/child/web_scheduler_impl.cc b/components/scheduler/child/web_scheduler_impl.cc
index 9d8aac9eab6d37cee7c7443cf0dfc099cbc6c973..5bf3dab324b0eb996d56442bfa0585ff1acfc1b8 100644
--- a/components/scheduler/child/web_scheduler_impl.cc
+++ b/components/scheduler/child/web_scheduler_impl.cc
@@ -15,11 +15,13 @@ WebSchedulerImpl::WebSchedulerImpl(
     ChildScheduler* child_scheduler,
     scoped_refptr<SingleThreadIdleTaskRunner> idle_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> loading_task_runner,
-    scoped_refptr<base::SingleThreadTaskRunner> timer_task_runner)
+    scoped_refptr<base::SingleThreadTaskRunner> timer_task_runner,
+    scoped_refptr<base::SingleThreadTaskRunner> shutdown_task_runner)
     : child_scheduler_(child_scheduler),
       idle_task_runner_(idle_task_runner),
       loading_task_runner_(loading_task_runner),
-      timer_task_runner_(timer_task_runner) {
+      timer_task_runner_(timer_task_runner),
+      shutdown_task_runner_(shutdown_task_runner) {
 }
 
 WebSchedulerImpl::~WebSchedulerImpl() {
@@ -104,4 +106,22 @@ void WebSchedulerImpl::postTimerTask(
       base::TimeDelta::FromMilliseconds(delayMs));
 }
 
+void WebSchedulerImpl::postShutdownTask(
+    const blink::WebTraceLocation& web_location,
+    blink::WebThread::Task* task,
+    long long delayMs) {
+  DCHECK(shutdown_task_runner_);
+  scoped_ptr<blink::WebThread::Task> scoped_task(task);
+  tracked_objects::Location location(web_location.functionName(),
+                                     web_location.fileName(), -1, nullptr);
+  shutdown_task_runner_->PostDelayedTask(
+      location,
+      base::Bind(&WebSchedulerImpl::runTask, base::Passed(&scoped_task)),
+      base::TimeDelta::FromMilliseconds(delayMs));
+}
+
+void WebSchedulerImpl::preShutdown() {
+  child_scheduler_->PreShutdown();
+}
+
 }  // namespace scheduler