Auto merge of #12518 - creativcoder:swm-fix, r=nox

Remove expect calls in service worker manager thread

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An intermittent timeout was seen in #12516 , caused by panic of ServiceWorkerManager thread on reception of messages from resource thread. This PR amends things to not have the ServiceWorkerManager thread panic in such situation.
cc @jdm

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix ##12516 (github issue number if applicable).

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- [X] These changes do not require tests because its an "intermittent fix"

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