Fixed DOM traversal using NodeRenderingTraversal::parent().

This fix was accidentally dropped from
https://codereview.chromium.org/402603005/

R=esprehn@chromium.org,rbyers@chromium.org

Review URL: https://codereview.chromium.org/486713003

git-svn-id: svn://svn.chromium.org/blink/trunk@180654 bbb929c8-8fbe-4397-9dbb-9b2b20218538