LibWeb: Stub out a dummy window.getComputedStyle()

This just returns an empty CSSStyleDeclaration for now. The real thing
needs to be a live object that provides a view onto the computed style
of a given element. This is far from that, but it's something. :^)

Userland/Libraries/LibWeb/DOM/Window.h

@@ -72,6 +72,8 @@ public:
     Event const* current_event() const { return m_current_event; }
     void set_current_event(Event* event) { m_current_event = event; }
 
+    NonnullRefPtr<CSS::CSSStyleDeclaration> get_computed_style(DOM::Element&) const;
+
 private:
     explicit Window(Document&);