Have Node::enclosingLinkEventParentOrSelf() return an Element

Have Node::enclosingLinkEventParentOrSelf() return an Element so that the
caller does not have to do an unsafe-looking toElement() cast.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/417093006

git-svn-id: svn://svn.chromium.org/blink/trunk@179281 bbb929c8-8fbe-4397-9dbb-9b2b20218538