Treat inert nodes as aria-hidden instead of aria-disabled.

As per the spec change: http://html5.org/r/8209

BUG=304779

Review URL: https://codereview.chromium.org/51743003

git-svn-id: svn://svn.chromium.org/blink/trunk@161015 bbb929c8-8fbe-4397-9dbb-9b2b20218538