AUI-798 - AUI Dialog iFrame does not display loading mask when updating url

Hey Nate,

Attached is an update to the loading mask for the AUI Dialog iFrame.  This issue can be found at http://issues.liferay.com/browse/AUI-798.

What I found was that we show and hide the loading mask in the same method and from the UI it looks as though the loading mask never renders.  We don't need the loadingmask.hide() in _uiSetUri() since when the url get's updated, it goes through _defaultLoadIframeFn() which runs loadingmask.hide().

Please let me know if you have any questions.

Thanks!

- Jon Mak