Fix `MonoMac.WebKit.DomNode' does not contain definition `Value'

monomac/samples/SkinnableApp/MainWindowController.cs(52,52): Error CS1061: Type `MonoMac.WebKit.DomNode' does not contain a definition for `Value' and no extension method `Value' of type `MonoMac.WebKit.DomNode' could be found. Are you missing an assembly reference? (CS1061) (SkinnableApp)