Fix bug for IE frames where onload events don't fire.

Issue: http://code.google.com/p/google-webtoolkit/issues/detail?id=1720

Review at http://gwt-code-reviews.appspot.com/1353801

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9785 8db76d5a-ed1c-0410-87a9-c151d255dfc7