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
Not sure, I think it is better to test.
Here is updated example with `return descr && !descr.get && !descr.set;` fix instead of previous: http://jsfiddle.net/znq76pg9/1/