Properly resize iframe root layers

When a layer containing an iframe changes, we also need to resize the
root layer of the subpage. This ensures that content from the child
layer tree is masked to the new size.

Fixes #8301.

components/compositing/compositor.rs

@@ -791,8 +791,20 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         self.pipeline_details.remove(&pipeline_id);
     }
 
-    fn update_layer_if_exists(&mut self, pipeline_id: PipelineId, properties: LayerProperties)
+    fn update_layer_if_exists(&mut self,
+                              pipeline_id: PipelineId,
+                              properties: LayerProperties)
                               -> bool {
+        if let Some(subpage_id) = properties.subpage_pipeline_id {
+            match self.find_layer_with_pipeline_and_layer_id(subpage_id, LayerId::null()) {
+                Some(layer) => {
+                    *layer.bounds.borrow_mut() = Rect::from_untyped(
+                        &Rect::new(Point2D::zero(), properties.rect.size));
+                }
+                None => warn!("Tried to update non-existent subpage root layer: {:?}", subpage_id),
+            }
+        }
+
         match self.find_layer_with_pipeline_and_layer_id(pipeline_id, properties.id) {
             Some(existing_layer) => {
                 existing_layer.update_layer(properties);