layout: Attempt to lay out blocks in parallel if we can prove (via
`clear` or by establishing a block formatting context) that they are
unaffected by floats.

This results in a 2.78x parallel speedup over sequential on the attached
test case, `perf-acid1.html`.
layout: Implement the correct behavior for block formatting contexts
adjacent to floats per CSS 2.1 § 9.5.

Results in an 80% parallel improvement on YouTube.