[Background Sync] Add getTags()

Add getTags to the PeriodicSyncManager idl file and add a web test to
verify it works as expected.

Also update test expectations.

Bug: 925297
Change-Id: I7902474ad236803a40ac630100ecccebb0652a97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669306
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672177}
10 files changed