struct ViewRefControl

A ViewRefControl is the peer to a ViewRef. Their `reference`s are linked.

Like ViewRef, a ViewRefControl is a typed handle to an eventpair. Unlike

ViewRef, a ViewRefControl's handle is unique. Scenic uses this property

when it ties a ViewRefControl to a View, arranged to share fate. When a

View is destroyed, the associated destruction of its ViewRefControl

triggers an automatic `ZX_EVENTPAIR_PEER_CLOSED` signal sent to all ViewRef

holders; hence ViewRef holders may track View lifetime.

As part of View creation, the client creates a linked

ViewRef/ViewRefControl pair and hands the pair to Scenic (ViewRef is

described above). The client must not clone the ViewRefControl. It must

not remove or modify the ViewRefControl's capabilities; otherwise the View

is not created.