Dirty rects always contain full tiles with delegated rendering.

cc/resources/layer_updater.h

Index: cc/resources/layer_updater.h
diff --git a/cc/resources/layer_updater.h b/cc/resources/layer_updater.h
index 5d8eb89ad4e2b2295309a52e909a56ea1914a69f..e56d206a85787564594528002ad5c58485951ec4 100644
--- a/cc/resources/layer_updater.h
+++ b/cc/resources/layer_updater.h
@@ -49,6 +49,7 @@ class CC_EXPORT LayerUpdater : public base::RefCounted<LayerUpdater> {
   // painted opaque. If the layer is marked opaque in the updater, then this
   // region should be ignored in preference for the entire layer's area.
   virtual void PrepareToUpdate(gfx::Rect content_rect,
+                               gfx::Rect paint_rect,
                                gfx::Size tile_size,
                                float contents_width_scale,
                                float contents_height_scale,