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

There's no callback for the first `appendChild()`, due to lack of a shadow-host-including root that is a document.

For the second call to `appendChild()`, by the time the callbacks are called, `this.ownerDocument()` will be `document2`. This follows from the operations `appendChild()` does.

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