PortProvider previously used a `WeakMap<Window, Set<Channel>>` to keep
track of which ports had been requested from a particular source frame,
where the map keys came from `MessageEvent.source`. A problem with this
approach is that for messages from an iframe, `MessageEvent.source` refers to a
`WindowProxy` which retains the same identity across page navigations.
As a result if a guest iframe was navigated to a new page which also
embedded the client as a guest (or into which the client was
subsequently injected as a guest), the guest in the new document would
not be able to retrieve any ports.

Implement a fix whereby each call to `PortFinder.discover` generates a
random request ID which is sent with the initial port request plus any
re-sent requests. PortProvider then keeps track of the request IDs it
has seen and ignores messages with a previously seen request ID.

This change would also fix port discovery if the guest in a frame was
unloaded and later re-loaded.