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

Indeed, it's easy enough to do in userland, and I'm totally fine with keeping it there:
```javascript
function followSignal(followingAbortController, parentSignal) {
  // 1. If followingSignal’s aborted flag is set, then return.
  if (followingAbortController.aborted) {
    return;
  }
  // 2. If parentSignal’s aborted flag is set, then signal abort on followingSignal.
  if (parentSignal.aborted) {
    followingAbortController.abort();
    return;
  }
  // 3. Otherwise, add the following abort steps to parentSignal:
  const onAbort = () => {
    // 3.1. Signal abort on followingSignal.
    followingAbortController.abort();
  };
  parentSignal.addEventListener('abort', onAbort, { once: true });
  // Non-standard: if followingSignal becomes aborted for some other reason,
  // remove the abort steps from the parentSignal.
  followingAbortController.signal.addEventListener('abort', () => {
    parentSignal.removeEventListener('abort', onAbort);
  });
}
```
That said, I wouldn't mind having this exposed as `AbortController.prototype.follow(signal)` if there's enough interest. 🙂

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