Store ResizeObservation targets weakly, matching Blink and WebKit. A
ResizeObserver can be kept alive by the document while it has observed
targets, but the observation itself should not keep a removed target and
its subtree alive forever.
Prune dead observations before gathering active resize observations.
Snapshot the document observer list before pruning so unregistering an
observer cannot mutate the intrusive list being iterated. Keep gathered
active targets rooted while broadcasting callbacks, since an earlier
callback can remove a later target and trigger GC before delivery.
Expose an internals helper to force environment-bound test objects to be
treated as garbage. Add text coverage for both pruning a dead observer
during gather and GC during an earlier resize observer callback.
a36f8aa36d