Switch .prop to .attr so this fix actually works.
Add constant for the delay value.
Add null check in recomputeIsVisibleInWindow.

There are some circumstances where a view could get destroyed while
recomputeIsVisibleInWindow is happening, which could lead to childViews
going away in the middle of this loop.
Fix caching issue with SC.routes.informLocation.

Since location and informLocation really just represent a single property,
they both need to update the cached value for the opposite property.
Further changes to unregister child records when a record is unloaded.
Bumping to SC 1.9.0.6.