[css3-sizing] When collapsed margins are calculated in extrinsic sizing

We should specify that, in the extrinsic sizing calculation, margins
are calculated from the element they end up being attached to after
collapsing.  That is, in an example like this:

<div margin: 10px; height: 100px;>
  <div margin: 10px;>
    <div height: fill-avialable></div>
  </div>
</div>

The inner div should be 100px tall, because the other two margins
collapse to "outside" the definite-height grandparent.

~TJ