DevTools: emulate offline mode for SW sync API

This change introduces a way for the DevTools to emulate offline mode
for the service worker background sync API. Notifications are queued up
and dispatched once the service worker comes online.

Bug: 625798
Change-Id: Ie804b3699991b9b4cbc7035be8e1f3e03fb39596
Reviewed-on: https://chromium-review.googlesource.com/956099
Commit-Queue: Eugene Ostroukhov <eostroukhov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550844}
6 files changed