Wrap script/iframe/img/link/source IDL setters, setAttribute, the four
HTML-parsing entry points, fetch, XHR, and sendBeacon so extension stacks
are visible at the synchronous call site. Coordinate via an
extensionElements WeakSet and an extensionUrls Set so the existing
MutationObserver and PerformanceObserver paths drop the same resources.
Signed-off-by: Émile Ré <emile@probo.com>