This class can be used to monitor changes in font size. Instances will
dispatch a goog.dom.FontSizeMonitor.EventType.CHANGE event.
Example usage:
var fms = new goog.dom.FontSizeMonitor();
goog.events.listen(fms, goog.dom.FontSizeMonitor.EventType.CHANGE,
function(e) {
alert('Font size was changed');
});