Updated code as suggested by reviewers.

common/lib/xmodule/xmodule/js/src/video/06_video_progress_slider.js

@@ -127,12 +127,12 @@ function () {
         width = (100 * ((end - start) / params.duration)).toFixed(1);
 
         if (!this.videoProgressSlider.sliderRange) {
-            this.videoProgressSlider.sliderRange = $('<div />')
-                .addClass('ui-slider-range')
-                .addClass('ui-widget-header')
-                .addClass('ui-corner-all')
-                .addClass('slider-range')
-                .css({
+            this.videoProgressSlider.sliderRange = $('<div />', {
+                class: 'ui-slider-range ' +
+                       'ui-widget-header ' +
+                       'ui-corner-all ' +
+                       'slider-range'
+            }).css({
                 left: left + '%',
                 width: width + '%'
             });