script: Move the layout_wrapper outside of script.

This allows us to have ensure_data() and clear_data() functions on the TElement
trait, instead of hacking around it adding methods in random traits.

This also allows us to do some further cleanup, which I'd rather do in a
followup.

components/script_layout_interface/wrapper_traits.rs

@@ -87,6 +87,7 @@ pub trait LayoutNode: Debug + GetLayoutData + TNode {
     /// Returns the type ID of this node.
     fn type_id(&self) -> LayoutNodeType;
 
+    unsafe fn initialize_data(&self);
     unsafe fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData);
     unsafe fn take_style_and_layout_data(&self) -> OpaqueStyleAndLayoutData;