Fix dtd for collapse-property.

Review at: http://gwt-code-reviews.appspot.com/1580804/
Patch by: stephenh

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10937 8db76d5a-ed1c-0410-87a9-c151d255dfc7