This isn't quite correct in the case of iframes. This will set the base compositor layer correctly, but if an iframe has unrendered content, the background color will be sent to the root's compositor layer instead the iframe's.

Instead, you should add logic to `compositor_layer.rs` that will recursively look for the iframe with the correct id that was passed in via this message, and only set the background color if the ids match; otherwise, recurse on the layer's children. Then, call that method here rather than setting the layer's color directly.

You might also need to set `recomposite` to true so the updates will immediately be displayed to the screen.