Add aria-hidden state to layout panel rulers.

Contributed by tbroyer
Fixes issues 7646
Review at: https://gwt-code-reviews.appspot.com/1820807/

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11263 8db76d5a-ed1c-0410-87a9-c151d255dfc7