Fix for progressbar styling

Modified the styling for the progressbar as the last fix didn't seem to
work in latest updates of Chrome, where the progressbar in Umbraco 7.1.6 and Chrome Version 37.0.2062.103 m hadn't a full width.

Previously pull requests: https://github.com/umbraco/Umbraco-CMS/pull/420 and https://github.com/umbraco/Umbraco-CMS/pull/456