Add figcaption case at 'computeAccessibilityIsIgnored()'
to keep figcation infomation on accessibility tree.

BUG=None

Review URL: https://codereview.chromium.org/553033002

git-svn-id: svn://svn.chromium.org/blink/trunk@181681 bbb929c8-8fbe-4397-9dbb-9b2b20218538