[New Multicolumn] Stay out of the old multicol machinery for column-span:all.

Note that column-span:all isn't supported in the new implementation yet,
but while we wait for that to happen, the least thing we can do is to stay alive.
We're not going to use this code in the new implementation anyway, when we get there.

BUG=344360

Review URL: https://codereview.chromium.org/237673006

git-svn-id: svn://svn.chromium.org/blink/trunk@171688 bbb929c8-8fbe-4397-9dbb-9b2b20218538