Fix a problem that popups don't close on iPads.
Fixes issue 7596

Review at http://gwt-code-reviews.appspot.com/1810803

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11219 8db76d5a-ed1c-0410-87a9-c151d255dfc7