Incremental Style Recalc

This patch puts in the initial framework for incremental reflow. Nodes' styles
are no longer recalculated unless the node has changed.

I've been hacking on the general problem of incremental reflow for the past
couple weeks, and I've yet to get a full implementation that actually passes all
the reftests + wikipedia + cnn. Therefore, I'm going to try to land the different
parts of it one by one.

This patch only does incremental style recalc, without incremental flow
construction, inline-size bubbling, reflow, or display lists. Those will be coming
in that order as I finish them.

At least with this strategy, I can land a working version of incremental reflow,
even if not yet complete.

r? @pcwalton
Can you add a TODO mentioning that this is usually only necessary to account for sibling selectors? Eventually we should avoid doing this if not necessary.