Update trans-iframe.js

Remove the check for that.iframeObj.iframe.contentWindow before sending close; the utils.createIframe does not currently return a reference to the iframe so this check always fails and no close message is ever sent.

The post method set up in utils.createIframe already tests for iframe.contentWindow so this approach seems safe.

I see now that the iframe reference on which this depended was removed in #55, so this all appears to be the intention.