Handle deferred layout updates gracefully in SplitLayoutPanel resize logic.

Resubmitting with fixes for the CellViewSuite test breakage.

Review at http://gwt-code-reviews.appspot.com/1295804

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9606 8db76d5a-ed1c-0410-87a9-c151d255dfc7