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

@smaug---- pointed out document does not need to be passed, you can simply use `this.ownerDocument.defaultView`. He also pointed out that in Gecko newAncestor/oldAncestor are only passed for the root (where they'd mean the same as newParent/oldParent) and not the descendants.

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