[w3c/webcomponents] HTMLElement.prototype.offsetParent leaks a node inside a shadow tree (#497)

We need to match `offsetParent` so that when a node's container block is inside a shadow tree, we don't leak a node inside a shadow tree.

Chrome seems to return `null` in such cases.  It's somewhat undesirable in that this makes the feature useless (since you can't determine where your containing block is) but I don't have a strong opinion about this as long as we don't leak a node inside a shadow tree.

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