Re: [webcomponents] Replace attached/detached callbacks with insertedIntoDocument/removedFromDocument (#362)

@annevk I don't understand. The document seems 100% necessary. Consider this case:

```js
const p = document1.createElement("p");
const c = document1.createElement("custom-foo");
p.appendChild(c);

document2.body.appendChild(p);
```

In this case `p` is being inserted into `document2.body`. `inclusiveDescendant` is `c`. And `c`'s node document (i.e. the value of `this.ownerDocument` inside `connectedCallback`) is `document1`. But we are connecting it to `document2`. So we should be passing `document2`.

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