LibWeb: Make WindowObject's prototype immutable

While I was implementing IDL special operations, I noticed that for
global platform objects (e.g. WindowObject), the IDL spec makes their
prototype immutable.

https://heycam.github.io/webidl/#platform-object-setprototypeof

Userland/Libraries/LibWeb/Bindings/WindowObject.h

@@ -60,6 +60,8 @@ public:
         return *constructor;
     }
 
+    virtual bool internal_set_prototype_of(JS::Object* prototype) override;
+
 private:
     virtual void visit_edges(Visitor&) override;