Fix making an iframe visible when it was initially set to display:none.

When an iframe is created with display:none it sets the root layer to be
zero width and height. When updating the rect of the iframe from layout
send the entire rect rather than just the new origin, which handles the case
where the iframe has been made visible and now has a non-zero rect.

components/compositing/headless.rs

@@ -104,7 +104,7 @@ impl CompositorEventListener for NullCompositor {
 
             Msg::CreateOrUpdateBaseLayer(..) |
             Msg::CreateOrUpdateDescendantLayer(..) |
-            Msg::SetLayerOrigin(..) |
+            Msg::SetLayerRect(..) |
             Msg::AssignPaintedBuffers(..) |
             Msg::ChangeReadyState(..) |
             Msg::ChangePaintState(..) |