Check whether aria-expanded attribute is defined

This patch introduces new enum type, AccessibilityExpanded, 
for handling expanded and collapsed states to 
follow http://goo.gl/dVClpj.

BUG=98982
R=dmazzoni@chromium.org, mkwst@chromium.org

Review URL: https://codereview.chromium.org/679623002

git-svn-id: svn://svn.chromium.org/blink/trunk@184621 bbb929c8-8fbe-4397-9dbb-9b2b20218538