Auto merge of #14459 - heycam:seq-dom-depth, r=emilio

Fix current_dom_depth in sequential traversal. (fixes #14414)

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Since we pass around a reference to the one `PerLevelTraversalData` object in sequential traversal, we must update it after we process children.  Alternatively, we could switch to what the parallel traversal is doing and clone the object when passing it down.

r? @emilio

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- [X] `./mach build -d` does not report any errors
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- [X] These changes fix #14414 (github issue number if applicable).

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