Add textUnderElement of figcaption for figure in title
and add the case for figure in accessibilityDescription.
To follow the spec,
http://rawgit.com/w3c/aria/master/html-aam/html-aam.html#el-figure,
textUnderElement of figcaption is used for name of figure.

BUG=282035

Review URL: https://codereview.chromium.org/713873002

git-svn-id: svn://svn.chromium.org/blink/trunk@185267 bbb929c8-8fbe-4397-9dbb-9b2b20218538