Fix DCHECK during TileManager shutdown

cc/resources/image_raster_worker_pool.cc

Index: cc/resources/image_raster_worker_pool.cc
diff --git a/cc/resources/image_raster_worker_pool.cc b/cc/resources/image_raster_worker_pool.cc
index 6d11526b6a4a1fa92ecedd4a2af7c7ebbf89ddca..5800ce1f8c0cb3c0d92428140c19eed475a30a70 100644
--- a/cc/resources/image_raster_worker_pool.cc
+++ b/cc/resources/image_raster_worker_pool.cc
@@ -66,6 +66,11 @@ 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");