Fix LWF.prototype.destroy raises DOM Exception 8

LWF.prototype.destroy raises DOM Exception 8.

The exception is raised by this line.
https://github.com/gree/lwf/blob/faa9bc9a4f3f1bfa47d6e2721008516c4e38df08/coffee/webkitcss/lwf_webkitcss_resourcecache.coffee#L448

Because, LWF did not load script if data.useScript is false.
https://github.com/gree/lwf/blob/faa9bc9a4f3f1bfa47d6e2721008516c4e38df08/coffee/webkitcss/lwf_webkitcss_resourcecache.coffee#L125