[Background Sync] Add unregister().

Add unregister to the idl, update PeriodicSyncManager logic.
Add web_tests to verify unregister works as expected, and update
test expectations to reflect the updated interface.

Bug: 925297
Change-Id: I93795b2355033d244d7e56823df68dc0d3bffe86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670209
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672271}
10 files changed