Cells should only fill a CellWidget if they render exactly one topmost element, or the first child will push around the other children. This is a follow-on change to r9884.

Author: tbroyer, jlabanca

Review at http://gwt-code-reviews.appspot.com/1385806

Review by: pdr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9889 8db76d5a-ed1c-0410-87a9-c151d255dfc7