[ServiceWorker] Some code clean-up for content::ServiceWorkerContextClient

Using switch is the best option here to get compile time verification, and
we can catch it if WebServiceWorkerEventResult gains a new type later.

Here also replace some if-return with DCHECK.

BUG=None

Review-Url: https://codereview.chromium.org/2606303002
Cr-Commit-Position: refs/heads/master@{#442822}