Simplify the display list slightly

Now that WebRender uses an Iframe display item, we do not need the Noop
item for the non-WebRender path. We can simply reuse the Iframe display
item. Also remove the layer_id member from the LayeredItem struct, as
it is unused.

components/layout/webrender_helpers.rs

@@ -504,8 +504,7 @@ impl WebRenderDisplayItemConverter for DisplayItem {
             DisplayItem::LineClass(..) => {
                 println!("TODO DisplayItem::LineClass");
             }
-            DisplayItem::LayeredItemClass(..) |
-            DisplayItem::NoopClass(..) => {
+            DisplayItem::LayeredItemClass(..) => {
                 panic!("Unexpected in webrender!");
             }
             DisplayItem::BoxShadowClass(ref item) => {