layout: Unify the block-size computation for blocks and floats.

The float code was old and did not support most of CSS 2.1. So unifying
the two paths both simplifies code and improves functionality.

Improves the Reddit sidebar.
Merge pull request #3472 from pcwalton/unify-block-and-float-layout

layout: Unify the block-size computation for blocks and floats.

Reviewed-by: glennw