[Background Fetch] Rename "tag" to "id", and add some documentation

Change-Id: I9271490f9a5b68129797ba6b9062247d1b1b2c2a
Reviewed-on: https://chromium-review.googlesource.com/1135324
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575726}
6 files changed