[whatwg/dom] Sort out isTrusted (#338)

>From discussion with @smaug---- in https://github.com/whatwg/dom/pull/336 and IRC it seems that basically any user-agent dispatched event is trusted.

A notable exception is `click()`, which is untrusted.

But `postMessage()`-resulting message events, mutation events (as long as we keep them around), need to be trusted.

It seems there's not really a simple rule here, such as the JavaScript stack being empty (mutation events fail that) or the user agent dispatching the event (`click()`).

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