[New Multicolumn] Make fast/multicol/overflow-content.html pass in the new implementation.

Insert a break at end of content on our own in the multicol code, to make sure
that overflow is properly accounted for.

BUG=369954

Review URL: https://codereview.chromium.org/263083009

git-svn-id: svn://svn.chromium.org/blink/trunk@173531 bbb929c8-8fbe-4397-9dbb-9b2b20218538