class SetWeak

Sets the current [`fuchsia.sysmem2/Node`] and all child `Node`(s)

created after this message to weak, which means that a client's `Node`

client end (or a child created after this message) is not alone

sufficient to keep allocated VMOs alive.

All VMOs obtained from weak `Node`(s) are weak sysmem VMOs. See also

`close_weak_asap`.

This message is only permitted before the `Node` becomes ready for

allocation (else the server closes the channel with `ZX_ERR_BAD_STATE`):

* `BufferCollectionToken`: any time

* `BufferCollection`: before `SetConstraints`

* `BufferCollectionTokenGroup`: before `AllChildrenPresent`

Currently, no conversion from strong `Node` to weak `Node` after ready

for allocation is provided, but a client can simulate that by creating

an additional `Node` before allocation and setting that additional

`Node` to weak, and then potentially at some point later sending

`Release` and closing the client end of the client's strong `Node`, but

keeping the client's weak `Node`.

Zero strong `Node`(s) and zero strong VMO handles will result in buffer

collection failure (all `Node` client end(s) will see

`ZX_CHANNEL_PEER_CLOSED` and all `close_weak_asap` `client_end`(s) will

see `ZX_EVENTPAIR_PEER_CLOSED`), but sysmem (intentionally) won't notice

this situation until all `Node`(s) are ready for allocation. For initial

allocation to succeed, at least one strong `Node` is required to exist

at allocation time, but after that client receives VMO handles, that

client can `BufferCollection.Release` and close the client end without

causing this type of failure.

This implies [`fuchsia.sysmem2/Node.SetWeakOk`] as well, but does not

imply `SetWeakOk` with `for_children_also` true, which can be sent

separately as appropriate.