Make <dialog> ARIA-aware.

With this patch:
1. Inert nodes behave like 'aria-disabled' is true.
2. <dialog> has a default role of 'dialog'

As according to the spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#wai-aria

BUG=264959

Review URL: https://chromiumcodereview.appspot.com/22903038

git-svn-id: svn://svn.chromium.org/blink/trunk@156769 bbb929c8-8fbe-4397-9dbb-9b2b20218538