[New Multicolumn] Avoid freeze if column count is 1 and there's a forced break.

If we don't have more columns available than the number of forced
breaks, we're going to have to allow overflowing columns. No matter
how tall we try to make the column set, there will be overflowing
columns, so just give up. This used to work as long as column count
was 2 or greater, but there was an unnecessary check that prevented it
from working if column count was 1. This was especially bad since we
actually stop counting forced breaks when we reach the number of
columns available.

BUG=359536

Review URL: https://codereview.chromium.org/223743002

git-svn-id: svn://svn.chromium.org/blink/trunk@170795 bbb929c8-8fbe-4397-9dbb-9b2b20218538