[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}
diff --git a/AUTHORS b/AUTHORS
index 0fff09e..712cbde 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -730,6 +730,7 @@
 William Xie <[email protected]>
 Xiang Long <[email protected]>
 Xiangze Zhang <[email protected]>
+Xiaofeng Zhang <[email protected]>
 Xiaolei Yu <[email protected]>
 Xiaoyin Liu <[email protected]>
 Xinchao He <[email protected]>