LibWeb: Invalidate style after CSSStyleSheet.{insert,remove}Rule()

When rules are inserted or removed via the CSSOM API, we now invalidate
document style to ensure that any changes made are reflected.

1% progression on ACID3. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -73,6 +73,7 @@ class StringStyleValue;
 class StyleComputer;
 class StyleProperties;
 class StyleSheet;
+class StyleSheetList;
 class StyleValue;
 class StyleValueList;
 class Supports;