[w3c/ServiceWorker] update() method algorithm needs correction (#1153)

Filed through https://github.com/w3c/ServiceWorker/pull/1152.

update() method should return a promise and go async. Currently, it does so when the algorithm execution reaches Run Job algorithm. In order to maintain this flow, we need to move the guarding steps in update() method to Update algorithm or put some equivalent fix.

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https://github.com/w3c/ServiceWorker/issues/1153