Previously this setting would just invalidate the whole CSS tree and
thereby hopefully avoid all cache usage.
Now, we actually don't cause extra invalidations anymore, but instead
avoid ever inserting anything into the cache when this setting is set.
This essentially copies the previous cache implementation. With one
caveat: It is now attached to and maintained by the CssNode, not by the
CssStyle.
And this is important because styles may be reused in incompatible
situations which would cause cache collisions and lead to broken CSS in
weird situations.
For now, the split out style cache doesn't cache anything. This is
mostly to make sure that bisections of wrong caching behavior will
bisect down to the commit that actually adds caching.
