class SetWeakOk

This indicates to sysmem that the client is prepared to pay attention to

`close_weak_asap`.

If sent, this message must be before

[`fuchsia.sysmem2/BufferCollection.WaitForAllBuffersAllocated`].

All participants using a weak [`fuchsia.sysmem2/BufferCollection`] must

send this message before `WaitForAllBuffersAllocated`, or a parent

`Node` must have sent [`fuchsia.sysmem2/Node.SetWeakOk`] with

`for_child_nodes_also` true, else the `WaitForAllBuffersAllocated` will

trigger buffer collection failure.

This message is necessary because weak sysmem VMOs have not always been

a thing, so older clients are not aware of the need to pay attention to

`close_weak_asap` `ZX_EVENTPAIR_PEER_CLOSED` and close all remaining

sysmem weak VMO handles asap. By having this message and requiring

participants to indicate their acceptance of this aspect of the overall

protocol, we avoid situations where an older client is delivered a weak

VMO without any way for sysmem to get that VMO to close quickly later

(and on a per-buffer basis).

A participant that doesn't handle `close_weak_asap` and also doesn't

retrieve any VMO handles via `WaitForAllBuffersAllocated` doesn't need

to send `SetWeakOk` (and doesn't need to have a parent `Node` send

`SetWeakOk` with `for_child_nodes_also` true either). However, if that

same participant has a child/delegate which does retrieve VMOs, that

child/delegate will need to send `SetWeakOk` before

`WaitForAllBuffersAllocated`.

+ request `for_child_nodes_also` If present and true, this means direct

child nodes of this node created after this message plus all

descendants of those nodes will behave as if `SetWeakOk` was sent on

those nodes. Any child node of this node that was created before this

message is not included. This setting is "sticky" in the sense that a

subsequent `SetWeakOk` without this bool set to true does not reset

the server-side bool. If this creates a problem for a participant, a

workaround is to `SetWeakOk` with `for_child_nodes_also` true on child

tokens instead, as appropriate. A participant should only set

`for_child_nodes_also` true if the participant can really promise to

obey `close_weak_asap` both for its own weak VMO handles, and for all

weak VMO handles held by participants holding the corresponding child

`Node`(s). When `for_child_nodes_also` is set, descendent `Node`(s)

which are using sysmem(1) can be weak, despite the clients of those

sysmem1 `Node`(s) not having any direct way to `SetWeakOk` or any

direct way to find out about `close_weak_asap`. This only applies to

descendents of this `Node` which are using sysmem(1), not to this

`Node` when converted directly from a sysmem2 token to a sysmem(1)

token, which will fail allocation unless an ancestor of this `Node`

specified `for_child_nodes_also` true.