[New Multicolumn] Make floats affect heights correctly.

Don't let overhanging floats bleed into sibling column sets or into the parent block flow.

Let RenderFlowThread establish a new block formatting context, so that its
height encompasses all descendant floats.

BUG=361551

Review URL: https://codereview.chromium.org/242203003

git-svn-id: svn://svn.chromium.org/blink/trunk@171996 bbb929c8-8fbe-4397-9dbb-9b2b20218538