bgsync: Simplify now that service worker is on the UI thread (pt 2).

- BackgroundSyncProxy does not need to do any cross-thread work.
- BackgroundSyncNetworkObserver lives on the UI thread.

Bug: 1138155
Change-Id: Ie731f487c37c45c8100169d09c301cd93d484fe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144630


Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918761}