Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/html_viewer/web_scheduler_impl.h

 // 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.
 
 #ifndef COMPONENTS_HTML_VIEWER_WEB_SCHEDULER_IMPL_H_
 #define COMPONENTS_HTML_VIEWER_WEB_SCHEDULER_IMPL_H_
 
 #include "third_party/WebKit/public/platform/WebScheduler.h"
 #include "third_party/WebKit/public/platform/WebThread.h"
 #include "third_party/WebKit/public/platform/WebTraceLocation.h"
 
 namespace html_viewer {
 
 class WebSchedulerImpl : public blink::WebScheduler {
  public:
   explicit WebSchedulerImpl(
       scoped_refptr<base::SingleThreadTaskRunner> task_runner);
   virtual ~WebSchedulerImpl();
 
  private:
   // blink::WebScheduler overrides.
   virtual void postIdleTask(const blink::WebTraceLocation& location,
                             blink::WebThread::IdleTask* task);
   virtual void postLoadingTask(const blink::WebTraceLocation& location,
                                blink::WebThread::Task* task);
   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);
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(WebSchedulerImpl);
 };
 
 }  // namespace html_viewer
 
 #endif  // COMPONENTS_HTML_VIEWER_WEB_SCHEDULER_IMPL_H_