LibWeb: Serialize resolved CSS values in dedicated serialization mode

This doesn't actually do anything differently yet, but it will in the
very next commit.

Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.h

@@ -38,6 +38,8 @@ private:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
+    virtual bool computed_flag() const override { return true; }
+
     RefPtr<CSSStyleValue const> style_value_for_property(Layout::NodeWithStyle const&, PropertyID) const;
 
     GC::Ref<DOM::Element> m_element;