Fixes a bug in Webkit where dynamically added STYLE tags are not always processed

STYLE tags (for example, those created by qx.ui.table.cellrenderer.Abstract) are not always processed by Webkit (affects at least Chrome 18 and Safari on OSX, Chrome 18 on Windows).

This webkit-specific patch triggers webkit to re-evaluate the STYLE tag's rules and apply them to the page correctly.

Relates to bug http://bugs.qooxdoo.org/show_bug.cgi?id=6348