Adds accessibility role for figure tag and handles its children as a group.
figure tag can have several children.
This patch makes its children a group for accessibility role.

BUG=282040

Review URL: https://codereview.chromium.org/515843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181203 bbb929c8-8fbe-4397-9dbb-9b2b20218538