Disallow relayout roots inside multicol.

It's just too complicated to figure out if it's safe to let a descendant of a
multicol container be a relayout root, so disallow it.

BUG=515260
R=jchaffraix@chromium.org,leviw@chromium.org

Review URL: https://codereview.chromium.org/1271513004

git-svn-id: svn://svn.chromium.org/blink/trunk@200034 bbb929c8-8fbe-4397-9dbb-9b2b20218538
7 files changed