box: Use macros to reduce code duplication
Always use the border-box for Box.position.size

Fixes #1674.
reftest: Allow free-form comments
Add a reftest for #1674
Add a simple iframe reftest (disabled for now)

Disabled due to iframe crashiness.
Rename Box.position to border_box
auto merge of #1699 : kmcallister/servo/border-box, r=pcwalton

I think this is the only change necessary to make block and inline boxes consistent.  But I'm finding it hard to test due to other bugs (#1696, #1697, #1698).