Fix for error in Firefox because of workaround for Safari

Fix for https://github.com/Polymer/platform/issues/94
RegExp based on user agent examples from http://myip.ms/view/comp_browsers/3458/Safari_8.html beause I do not have any available machine with Safari 8
My concern was that a browser might do some other crazy thing where it implements things as magic data properties.

Still. LGTM with `return descr && !descr.get && !descr.set;`. Care to update the PR?