class CreateChild

Create a child [`fuchsia.sysmem2/BufferCollectionToken`]. Only one child

(including its children) will be selected during allocation (or logical

allocation).

Before passing the client end of this token to

[`fuchsia.sysmem2/Allocator.BindSharedCollection`], completion of

[`fuchsia.sysmem2/Node.Sync`] after

[`fuchsia.sysmem2/BufferCollectionTokenGroup.CreateChild`] is required.

Or the client can use

[`fuchsia.sysmem2/BufferCollectionTokenGroup.CreateChildrenSync`] which

essentially includes the `Sync`.

Sending CreateChild after AllChildrenPresent is not permitted; this will

fail the group's subtree and close the connection.

After all children have been created, send AllChildrenPresent.

+ request `token_request` The server end of the new token channel.

+ request `rights_attenuation_mask` If ZX_RIGHT_SAME_RIGHTS, the created

token allows the holder to get the same rights to buffers as the

parent token (of the group) had. When the value isn't

ZX_RIGHT_SAME_RIGHTS, the value is interpretted as a bitmask with 0

bits ensuring those rights are attentuated, so 0xFFFFFFFF is a synonym

for ZX_RIGHT_SAME_RIGHTS. The value 0 is not allowed and intentionally

causes subtree failure.