Fix for issue 4360. There was a subtle state bug in LayoutCommand that
was causing this. This patch includes a test for LayoutPanel that checks
for this error.
Review: http://gwt-code-reviews.appspot.com/132803

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7389 8db76d5a-ed1c-0410-87a9-c151d255dfc7