Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/webthread_impl_for_worker_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/child/webthread_impl_for_worker_scheduler.h"
 
 #include "base/bind.h"
 #include "base/synchronization/waitable_event.h"
 #include "components/scheduler/child/scheduler_message_loop_delegate.h"
 #include "components/scheduler/child/web_scheduler_impl.h"
 #include "components/scheduler/child/worker_scheduler_impl.h"
 #include "third_party/WebKit/public/platform/WebTraceLocation.h"
 
 namespace scheduler {
 
 WebThreadImplForWorkerScheduler::WebThreadImplForWorkerScheduler(
     const char* name)
     : thread_(new base::Thread(name)) {
   thread_->Start();
 
   base::WaitableEvent completion(false, false);
   thread_->message_loop()->PostTask(
       FROM_HERE, base::Bind(&WebThreadImplForWorkerScheduler::InitOnThread,
                             base::Unretained(this), &completion));
   completion.Wait();
 }
 
 WebThreadImplForWorkerScheduler::~WebThreadImplForWorkerScheduler() {
   base::WaitableEvent completion(false, false);
-  // We need to post the shutdown task on the scheduler's task queue or tasks
-  // posted on the worker scheduler may not get run when the thread is deleted.
-  TaskRunner()->PostTask(
+  // We need to post the shutdown task on the scheduler's shutdown task queue or
+  // other (shutdown) tasks posted on the worker scheduler may not get run when
+  // the thread is deleted.
+  worker_scheduler_->ShutdownTaskRunner()->PostTask(
       FROM_HERE, base::Bind(&WebThreadImplForWorkerScheduler::ShutDownOnThread,
                             base::Unretained(this), &completion));
   completion.Wait();
 
   thread_->Stop();
 }
 
 void WebThreadImplForWorkerScheduler::InitOnThread(
     base::WaitableEvent* completion) {
   worker_scheduler_ = WorkerScheduler::Create(thread_->message_loop());
   worker_scheduler_->Init();
   task_runner_ = worker_scheduler_->DefaultTaskRunner();
   idle_task_runner_ = worker_scheduler_->IdleTaskRunner();
   web_scheduler_.reset(new WebSchedulerImpl(
       worker_scheduler_.get(), worker_scheduler_->IdleTaskRunner(),
       worker_scheduler_->DefaultTaskRunner(),
-      worker_scheduler_->DefaultTaskRunner()));
+      worker_scheduler_->DefaultTaskRunner(),
+      worker_scheduler_->ShutdownTaskRunner()));
   completion->Signal();
 }
 
 void WebThreadImplForWorkerScheduler::ShutDownOnThread(
     base::WaitableEvent* completion) {
   task_runner_ = nullptr;
   idle_task_runner_ = nullptr;
   web_scheduler_.reset();
   worker_scheduler_.reset();
   completion->Signal();
@@ skipping 26 matching lines @@
     base::MessageLoop::TaskObserver* observer) {
   worker_scheduler_->AddTaskObserver(observer);
 }
 
 void WebThreadImplForWorkerScheduler::RemoveTaskObserverInternal(
     base::MessageLoop::TaskObserver* observer) {
   worker_scheduler_->RemoveTaskObserver(observer);
 }
 
 }  // namespace scheduler