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

web/compatibility.js

@@ -92,6 +92,13 @@ if (typeof PDFJS === 'undefined') {
   window.Float64Array = TypedArray;
 })();
 
+// URL = URL || webkitURL
+(function normalizeURLObject() {
+  if (!window.URL && window.webkitURL) {
+    window.URL = window.webkitURL;
+  }
+})();
+
 // Object.create() ?
 (function checkObjectCreateCompatibility() {
   if (typeof Object.create !== 'undefined')