Fixing a bug in CellTree where the Show More button doesn't work for the root node.

Issue: 5547

Review at http://gwt-code-reviews.appspot.com/1420801

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9997 8db76d5a-ed1c-0410-87a9-c151d255dfc7