[whatwg/dom] Restore active custom element constructor map entries after construction (PR #1494)

Create an element sets the active custom element constructor map entry for the constructor it is about to construct, then removed it once construction finished. A re-entrant construction of the same constructor for a different registry therefore clobbered the outer entry, and its removal left the outer construction to resolve against the wrong registry.

Save the previous entry before setting it and restore it afterwards, so the outer registry survives a re-entrant construction. This mirrors the same change to the HTML element constructor and upgrade an element.

See https://github.com/whatwg/html/pull/12696.

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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1494

-- Commit Summary --

  * Restore active custom element constructor map entries after construction

-- File Changes --

    M dom.bs (14)

-- Patch Links --

https://github.com/whatwg/dom/pull/1494.patch
https://github.com/whatwg/dom/pull/1494.diff

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