class RegisterScopedViewTreeWatcher

Sets up a geometry `ViewTreeWatcher` channel which reports the view tree

geometry for a specific view, identified by `context_view`. The data

received describes the `context_view` and its descendant views.

This method may be called multiple times to obtain multiple geometry

`ViewTreeWatcher`s. Typical usage is to obtain just one geometry

`ViewTreeWatcher`.

Client can freely disconnect this `Registry` endpoint after this method

returns and the `ViewTreeWatcher` endpoint will remain active.

Flow control. The caller is allowed at most one in-flight call at a

time. Subsequent calls must wait until the acknowledgment returns.

Non-compliance results in channel closure.

Client synchronization. The method response signifies that the server

has processed the channel endpoint. A client can synchronize its next

actions based on this guarantee (for example, trigger a change in view

geometry without the change "getting lost").

View lifecycle. The view represented by `context_view` must first be

created. Otherwise, `geometry` channel's server endpoint is closed.

+ request `context_view` a view's identity, which serves as the root of

a view tree

+ request `geometry` a channel to observe `context_view`'s view tree

- response acknowledgement that `geometry` is connected to the server