Instead of traversing the entire DOM subtrees and marking nodes for
style update, this patch adds a new mechanism where we can mark a
subtree root as "entire subtree needs style update".
A new pass in Document::update_style() then takes care of coalescing
all these invalidations in a single traversal of the DOM.
This shaves *minutes* of loading time off of https://wpt.fyi/ subpages.
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