[New Multicolumn] Lay out flow thread again when multicol container height changes.

The column height may have to be adjusted when the multicol
container's height changes, so we need to descend into the
RenderMultiColumnFlow thread child of the RenderBlockFlow multicol
container when this happens.

BUG=359944

Review URL: https://codereview.chromium.org/225463002

git-svn-id: svn://svn.chromium.org/blink/trunk@170896 bbb929c8-8fbe-4397-9dbb-9b2b20218538