Auto merge of #8669 - bholley:generalize_layout, r=pcwalton

Generalize layout to operate on arbitrary DOMs

This gets us to where we need to be in order to write a Gecko implementation of the layout wrapper and have things Just Work.

Note that this is somewhat more than we need for just running the style system. But there wasn't a clean place to cut, and I thought it was a good idea to just do a complete job on it now, which may save us effort and mismatches in the long run.

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