Fix doubling noncontent_height in float flow
auto merge of #739 : ILyoan/servo/acid1_margin, r=metajack

In `assign_height_inorder_float()`, `base.position.size.height` already counted padding and border, so adding `noncontent_height` to `height` would result in doubling counted padding and border.

By this patch acid1 would be displayed more accurately.

Not sure but maybe width has the same problem, I'll investigate it.