Restore restyle damage computation but disable reflow pruning for now

This fixes the computation of restyle damage on `color-change-text.html`, which can be seen with `RUST_LOG=servo::layout::layout_task`.

However we can't prune the layout traversals yet, because we don't reuse `Flow` objects between reflows, so we have no old values to fall back to.

I think this used to work because `FlowContexts` (as they were called then) were stored in a DOM node's `LayoutData` and reused.  But it's possible that it never really worked, and my testing when I landed the restyle damage code was insufficient (I didn't understand the layout code nearly as well back then).

r? @pcwalton