Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/renderer/webthread_impl_for_renderer_scheduler.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/scheduler/renderer/webthread_impl_for_renderer_scheduler.h"
 
 #include "components/scheduler/child/web_scheduler_impl.h"
 #include "components/scheduler/renderer/renderer_scheduler.h"
 #include "third_party/WebKit/public/platform/WebTraceLocation.h"
 
 namespace scheduler {
 
 WebThreadImplForRendererScheduler::WebThreadImplForRendererScheduler(
     RendererScheduler* scheduler)
     : web_scheduler_(new WebSchedulerImpl(scheduler,
                                           scheduler->IdleTaskRunner(),
                                           scheduler->LoadingTaskRunner(),
-                                          scheduler->TimerTaskRunner())),
+                                          scheduler->TimerTaskRunner(),
+                                          scheduler->ShutdownTaskRunner())),
       task_runner_(scheduler->DefaultTaskRunner()),
       idle_task_runner_(scheduler->IdleTaskRunner()),
       scheduler_(scheduler),
       thread_id_(base::PlatformThread::CurrentId()) {
 }
 
 WebThreadImplForRendererScheduler::~WebThreadImplForRendererScheduler() {
 }
 
 blink::PlatformThreadId WebThreadImplForRendererScheduler::threadId() const {
@@ skipping 23 matching lines @@
     base::MessageLoop::TaskObserver* observer) {
   scheduler_->AddTaskObserver(observer);
 }
 
 void WebThreadImplForRendererScheduler::RemoveTaskObserverInternal(
     base::MessageLoop::TaskObserver* observer) {
   scheduler_->RemoveTaskObserver(observer);
 }
 
 }  // namespace scheduler