Re: [mediacapture-handle] Expose a MessagePort using Capture Handle (#70)

I like the minimal API in https://github.com/w3c/mediacapture-handle/issues/70#issuecomment-1406345920 on the controller, assuming `postMessage` is modeled on the one from [Window](https://html.spec.whatwg.org/multipage/nav-history-apis.html#the-window-object:dom-window-postmessage): 
```js
partial interface CaptureController {
  undefined postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
  undefined postMessage(any message, optional WindowPostMessageOptions options = {});
```
That way, apps have `targetOrigin` to deal with navigation in the capturee.

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