Auto merge of servo#16629 - emilio:dirty-doc, r=heycam

Bug 1336863: Ensure we don't restyle with outdated rules.

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ports/geckolib/glue.rs

@@ -178,6 +178,7 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
     }
 
     let per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow();
+    debug_assert!(!per_doc_data.stylesheets.has_changed());
 
     let token = RecalcStyleOnly::pre_traverse(element, &per_doc_data.stylist, traversal_flags);
     if !token.should_traverse() {