[Extensions] Replace a SW test with one that uses a background SW.

Service Worker tests which use Extension APIs will no longer work
once we restrict API bindings to scripts specified in the
extension's manifest. The CL replaces the old test with a new one.

Bug: 961836
Change-Id: Id8f96d920d0c6c275254561c86aaacefa2b56e2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614686
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661882}