Garbage collection means that the worklets might drop after the script
head has been cleaned up. The worklet now caches the thread pool in the
DOM object itself which should prevent it from needing to access script
thread TLS when being cleaned up. The value is stored as a OnceCell to
maintain the same lazy thread pool creation pattern as before.
Fixes#25838.
Fixes#25258.
* Update web-platform-tests to revision b'50e91f3cfd1140cafc188516488f2dca289239af'
* Delete flaky result for css2/linebox/vertical-align-baseline-002
* Delete flaky result for html/rendering/bindings/the-textarea-element-0/cols-default.html.ini
* Delete flaky result for CSS2/lists/list-style-applies-to-001.xht.ini
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Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>