Put URL = URL || webkitURL in compatibility.js

Declares the URL variable globally. If the feature is not
supported, the variable will still be declared, but have the
"undefined" value.

Supported by:

- Firefox 4
  Firefox 21 in Web worker

- Chrome 8  (prefixed as webkitURL), 23+ unprefixed
  Chrome 10 (prefixed as webkitURL) in Web Worker, 23+ unprefixed

- Opera 15
  Opera 15 in Web Worker

- Internet Explorer 10
  Internet Explorer 11 in Web Worker

- Safari 6 (prefixed as webkitURL)
  Safari 6 (prefixed as webkitURL) in Web Worker