[resize-observer] ResizeObserver callback order.

The specification has a well-defined callback order, however it requires keeping alive observers for too long (see this firefox bug).

It seems that WebKit is doing the same thing I'm going to do in Gecko (add to the Document.resizeObservers slot on observe(), and remove on the last unobserve() call).

It seems Blink uses a hash set so I don't even know if the iteration order there is well defined for them.

I'm going to align with WebKit for now as it seems simpler to implement and doesn't create an insert-only list / set.

Doing this creates an issue, which is that the observers can be removed when delivering observations. This was already a problem for additions, but explicitly copying the contents of the slot before iteration in the spec may be clearer.

cc @smfr @atotic

Start by locating the ResizeObserver specification text around the Document.resizeObservers slot and the observe(), unobserve(), and observation-delivery steps. Compare the proposed WebKit-aligned lifetime and callback iteration behavior, then update the specification so removals during delivery and callback order are defined; done means the intended algorithm is unambiguous.