End TreeBuilder's reliance on DOM.
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Fixed h5e's TreeBuilder so that it does not use `same_tree` and `has_parent_node` methods: d8c2ea5cb6
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18056)
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