Implementation of close event of MessagePort API

[nononokam]

Proposal

There was no timely and reliable way to detect when a MessagePort becomes disentangled, which made it difficult to release resources associated with ports.
We proposed to add a close event to MessagePort API.
The close event has landed to the HTML spec. Chrome has already completed implementation and it’s close to ship.

Initial issue: whatwg/html#1766
Explainer: https://github.com/fergald/explainer-messageport-close
HTML spec: https://html.spec.whatwg.org/multipage/web-messaging.html#disentangle
Chrome status: https://chromestatus.com/feature/5086799659794432
Mozilla standards position: mozilla/standards-positions#914
WebKit standards position: WebKit/standards-positions#271
HTML spec PR: whatwg/html#9933

Primary contacts: @rakina, @mingyc, @nononokam