fix(ext/web): make MessageEvent.ports a frozen array (#34773)
## What
`MessageEvent.ports` is specified as a `FrozenArray<MessagePort>`, so
the
array exposed by the event must be read-only. The constructor assigned a
plain mutable array, so `event.ports` could be mutated — e.g.
`event.ports.push(...)` silently succeeded instead of throwing a
`TypeError`.
This freezes the array in both branches of the `MessageEvent`
constructor (empty and populated). Message delivery in
`ext/web/13_message_port.js` constructs the event via the same
constructor (`new MessageEvent("message", { ports })`), so delivered
events get a frozen `ports` array too.
## Spec
[HTML — the MessageEvent
interface](https://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interface):
`readonly attribute FrozenArray<MessagePort> ports;`
## WPT subtests now passing
`webmessaging/Channel_postMessage_ports_readonly_array.any.html` and
`.any.worker.html`:
- *The postMessage() method - Make new ports into a read only array.*
`tests/wpt/runner/expectations/webmessaging.json` flipped from `false`
to
`true` for both files.
## Root cause
`MessageEvent` in `ext/web/02_event.js` stored `this.ports` as a plain
(non-frozen) array.
