[New Multicolumn] Use the term "content run" in favor of "forced break".

You get one content run even without any forced breaks.

Documentation improvements. In particular for flow thread layout and
column balancing.

Also don't assume that a column set starts at flow thread block offset 0.
This is only true for the first set. So far we only have one set, but this
will soon change, with the introduction of support for column-span:all.
Check with the flow thread portion rectangle to get the correct start offset.

Review URL: https://codereview.chromium.org/267373005

git-svn-id: svn://svn.chromium.org/blink/trunk@173879 bbb929c8-8fbe-4397-9dbb-9b2b20218538