LibWeb: Add Document::invalidate_layout()

This function allows you to throw away the entire layout tree if that's
something you want to do.

It's certainly not super cheap to reconstruct, but hey, who am I to
tell you what to do? :^)

Libraries/LibWeb/DOM/Document.h

@@ -106,6 +106,7 @@ public:
 
     void layout();
     void force_layout();
+    void invalidate_layout();
 
     void update_style();
     void update_layout();