cc: Give non-drawing layers that are rasterized a lower priority.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index c32c208ff13c6c885098244610ff9c81026912a0..fc49c58fd865a37da75d7520934aaa78d6395335 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -564,6 +564,10 @@ class CC_EXPORT LayerImpl {
   // layers) and consumed by LayerTreeImpl::PushPropertiesTo during activation.
   bool needs_show_scrollbars_ : 1;
 
+  // This is set for layers that have a property because of which they are not
+  // drawn (singular transforms), but they can become visible soon (the property
+  // is being animated). For this reason, while these layers are not drawn, they
+  // are still rasterized.
   bool raster_even_if_not_in_rsll_ : 1;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);