service worker: Minor cleanup in WebServiceWorkerNetworkProvider.

- Make it a pure abstract class for clarity.
- Make the subclasses implement each method for clarity.
- Group the methods "for clients" together.
- Improve documentation.

Change-Id: Ib54826cc8081ae5a62de6dc66e8fc10567bf832a
Reviewed-on: https://chromium-review.googlesource.com/c/1469081
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631502}
8 files changed