Fix DCHECK during TileManager shutdown

cc/resources/image_raster_worker_pool.cc

 // 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.
 
 #include "cc/resources/image_raster_worker_pool.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/resources/resource.h"
 #include "third_party/skia/include/core/SkDevice.h"
 
@@ skipping 48 matching lines @@
 
 ImageRasterWorkerPool::ImageRasterWorkerPool(
     ResourceProvider* resource_provider, size_t num_threads)
     : RasterWorkerPool(resource_provider, num_threads) {
 }
 
 ImageRasterWorkerPool::~ImageRasterWorkerPool() {
   DCHECK_EQ(0u, image_tasks_.size());
 }
 
+void ImageRasterWorkerPool::Shutdown() {
+  RasterWorkerPool::Shutdown();
+  ScheduleRasterTasks(RootTask());
+}
+
 void ImageRasterWorkerPool::ScheduleTasks(RasterTask::Queue* queue) {
   TRACE_EVENT0("cc", "ImageRasterWorkerPool::ScheduleTasks");
 
   internal::WorkerPoolTask::TaskVector tasks;
 
   RasterWorkerPool::SetRasterTasks(queue);
 
   for (RasterTask::Queue::TaskVector::const_iterator it =
            raster_tasks().begin();
        it != raster_tasks().end(); ++it) {
@@ skipping 55 matching lines @@
   if (!was_canceled)
     task->DidRun();
 
   task->DidComplete();
   task->DispatchCompletionCallback();
 
   image_tasks_.erase(task);
 }
 
 }  // namespace cc