Region based multicol: support explicit column breaks

Merely supporting insertion of explicit (forced) column breaks in
the region based multicol implementation is really simple: just
hook up with what the CSS regions code is already doing.

However, there is one complication: column balancing. In order to
balance columns as nicely as possible when there are explicit
breaks, we need to figure out between which explicit breaks the
implicit breaks will occur (if there's room for any at all).

This is a merge of https://bugs.webkit.org/show_bug.cgi?id=123993
Reviewed in WebKit by David Hyatt.

BUG=336192

Review URL: https://codereview.chromium.org/143383002

git-svn-id: svn://svn.chromium.org/blink/trunk@166859 bbb929c8-8fbe-4397-9dbb-9b2b20218538