[New Multicolumn] balancer confused by content with top margins.

We need to set zero page logical height in LayoutState when column
height is unknown (when the columns haven't yet been
balanced). There's code that assumes that non-zero page height means
that page height is known. Lying about this makes the pagination code
believe that every top margin is adjacent to a column break, which
makes it eat and ignore all top margins.

This should be cleaned up, but it's easier to wait until the old
multicol code has been removed.

BUG=353587

Review URL: https://codereview.chromium.org/209863003

git-svn-id: svn://svn.chromium.org/blink/trunk@170335 bbb929c8-8fbe-4397-9dbb-9b2b20218538