Support percentage height

[SimonSapin]

As of #1988, BlockFlow::assign_height_block_base has code like this:

let mut candidate_height_iterator = CandidateHeightIterator::new(
    fragment.style(), None);

None should instead be Some(containing_block_height) when that height can be resolved at this point. That is, it’s not auto nor a percentage that refers to something that can not be resolved at this point.

This probably involves propagating non-auto height down the tree in an earlier top-down pass.

See in http://dev.w3.org/csswg/css2/visudet.html#propdef-height

[SimonSapin]

Same in BlockFlow::assign_height_float, but BlockFlow::calculate_abs_height_and_margins looks already correct.

[SimonSapin]

There is an errata on CSS2 on this: http://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.10.5a this is not reflected in the ED as of this writing, even though it should be :(