Avoid srcset resource download when higher density resource is in cache.

LayoutTests/fast/dom/HTMLImageElement/resources/currentSrcHelper.js

 function fileName(src) {
     var array = src.split('/');
     return array[array.length -1];
 };
 
 function currentSrcFileName(id) {
     var currentSrc = document.getElementById(id).currentSrc;
     return fileName(currentSrc);
 };
+
+function currentSrcFileNameNoParams(id) {
+    var currentSrc = document.getElementById(id).currentSrc;
+    var name = fileName(currentSrc).split('?');
+    return name[0];
+};