Re: [ServiceWorker] Sort out promises' chained reaction text (5aa0ca2)

[Upon fulfillment](http://www.w3.org/2001/tag/doc/promises-guide#upon-fulfillment) doesn't use the result of `p.then`, so the embedded `return` will be ignored, rather than leading to the fulfillment of the result of `addAll()`. This is another case where we want to wait for a promise to settle rather than doing anything asynchronously (https://github.com/w3ctag/promises-guide/issues/36), but here you could use [the WebIDL "perform some steps once a promise is settled" algorithm](https://heycam.github.io/webidl/#dfn-perform-steps-once-promise-is-settled). The WebIDL algorithm will also solve the problem that if any element of `responseBodyPromiseArray` rejects, that doesn't propagate to the result of `addAll()`.

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