Add LayoutTest for AX Role of dl element
The related CL:
https://codereview.chromium.org/609793002/

BUG=226505

Review URL: https://codereview.chromium.org/623693002

git-svn-id: svn://svn.chromium.org/blink/trunk@183937 bbb929c8-8fbe-4397-9dbb-9b2b20218538