LibWeb: Maintain list of allocated shadow roots in Document

Doing that will allow us to get a list of style sheets for each shadow
root from StyleComputer without having to traverse the entire tree in
upcoming changes.

Userland/Libraries/LibWeb/DOM/ShadowRoot.h

@@ -42,6 +42,8 @@ public:
     JS::NonnullGCPtr<WebIDL::ObservableArray> adopted_style_sheets() const;
     WebIDL::ExceptionOr<void> set_adopted_style_sheets(JS::Value);
 
+    virtual void finalize() override;
+
 protected:
     virtual void visit_edges(Cell::Visitor&) override;