[whatwg/dom] AbortController, AbortSignal, and garbage collection (#920)

@smaug---- made me realize we could probably be a little bit more specific about what keeps what alive.

Thoughts:

* AbortController weakly references AbortSignal. The controller getting GC'd is fine, since it doesn't learn about results.
* AbortSignal cannot be collected when it has a controller and event listeners/abort algorithms. I think that means it needs a weak reference to its controller.

cc @jakearchibald @domenic 

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