Add AX role ,'AXDescriptionList', for dl element(re-land).

There are related patches from WebKit, the patch written by
James Craig<james@cookiecrook.com> wkrev.com/147520 and the
patch written by Mario Sanchez Prada  <mario.prada@samsung.com>
wkrev.com/161492.
And I modified them based on the current blink.

BUG=226505

Review URL: https://codereview.chromium.org/609793002

git-svn-id: svn://svn.chromium.org/blink/trunk@183062 bbb929c8-8fbe-4397-9dbb-9b2b20218538