Delivery logic for Object.observe
This CL has two parts: the first is the logic itself, whereby each observer callback is assigned
a "priority" number the first time it's passed as an observer to Object.observe(), and that
priority is used to determine the order of delivery.
The second part invokes the above logic as part of the API, when the JS stack winds down to
Added several tests via the API, as the delivery logic isn't testable from a JS test
(it runs after such a test would exit).