cc: Do bitmap conversion for RasterBuffer in the worker thread.

cc/resources/image_copy_raster_worker_pool.cc

Index: cc/resources/image_copy_raster_worker_pool.cc
diff --git a/cc/resources/image_copy_raster_worker_pool.cc b/cc/resources/image_copy_raster_worker_pool.cc
index 2a9b7f1eaac087729cb005978b5df888e9dbc888..d4705d14fabfd61e3bb9375b56147de40f9937d9 100644
--- a/cc/resources/image_copy_raster_worker_pool.cc
+++ b/cc/resources/image_copy_raster_worker_pool.cc
@@ -166,11 +166,13 @@ void ImageCopyRasterWorkerPool::CheckForCompletedTasks() {
   FlushCopies();
 }
 
-SkCanvas* ImageCopyRasterWorkerPool::AcquireCanvasForRaster(RasterTask* task) {
+RasterCanvas* ImageCopyRasterWorkerPool::AcquireCanvasForRaster(
+    RasterTask* task) {
   DCHECK_EQ(task->resource()->format(), resource_pool_->resource_format());
   scoped_ptr<ScopedResource> resource(
       resource_pool_->AcquireResource(task->resource()->size()));
-  SkCanvas* canvas = resource_provider_->MapImageRasterBuffer(resource->id());
+  RasterCanvas* canvas =
+      resource_provider_->MapImageRasterBuffer(resource->id());
   DCHECK(std::find_if(raster_task_states_.begin(),
                       raster_task_states_.end(),
                       RasterTaskState::TaskComparator(task)) ==