[OnionSoup] Define blink::ServiceWorkerStatusCode as "enum class'

This CL changes type of blink::ServiceWorkerStatusCode from 'enum' to
'enum class', then updates everywhere it is accessed.

BUG=789854

Change-Id: I837d8b0b06924f8f42fa7246231e654e364523f6
Reviewed-on: https://chromium-review.googlesource.com/1124102
Commit-Queue: Han Leon <[email protected]>
Reviewed-by: Makoto Shimazu <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#572469}
73 files changed