DirectInstallLinker should not immediately remove the script tag it has inserted into the IFRAME as this can fail in Webkit browsers

Review at http://gwt-code-reviews.appspot.com/1454802

Review by: unnurg@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10293 8db76d5a-ed1c-0410-87a9-c151d255dfc7