check existence of PercentLoaded function in other way

`this.getFlashElement().PercentLoaded` causes 'Object doesn't support this property or method' error in IE8.
`'PercentLoaded' in this.getFlashElement()` prevents this error.
Please find the below URL for the detail:
http://stackoverflow.com/questions/2192203/is-there-a-way-to-detect-flash-blockers#14649116