Fix DCHECK during TileManager shutdown

cc/resources/image_raster_worker_pool.h

 // Copyright 2013 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 CC_RESOURCES_IMAGE_RASTER_WORKER_POOL_H_
 #define CC_RESOURCES_IMAGE_RASTER_WORKER_POOL_H_
 
 #include "cc/resources/raster_worker_pool.h"
 
 namespace cc {
 
 class CC_EXPORT ImageRasterWorkerPool : public RasterWorkerPool {
  public:
   virtual ~ImageRasterWorkerPool();
 
   static scoped_ptr<RasterWorkerPool> Create(
       ResourceProvider* resource_provider, size_t num_threads) {
     return make_scoped_ptr<RasterWorkerPool>(
         new ImageRasterWorkerPool(resource_provider, num_threads));
   }
 
   // Overridden from RasterWorkerPool:
+  virtual void Shutdown() OVERRIDE;
   virtual void ScheduleTasks(RasterTask::Queue* queue) OVERRIDE;
 
  private:
   ImageRasterWorkerPool(ResourceProvider* resource_provider,
                         size_t num_threads);
 
   void OnRasterTaskCompleted(
       scoped_refptr<internal::RasterWorkerPoolTask> task, bool was_canceled);
 
   TaskMap image_tasks_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageRasterWorkerPool);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_IMAGE_RASTER_WORKER_POOL_H_