Tag should be released when a terminal event fires.

[paullewis]

We should define that a tag gets released whenever the backgroundfetch* events fire.

Filed against Chrome here: https://bugs.chromium.org/p/chromium/issues/detail?id=709921

[beverloo]

The important thing to define is when it fires— before or after the Service Worker event runs?

If the tag gets released before the event runs, it means that developers might be able to start another Background Fetch having the same tag in the event handler, but that comes at the cost of additional bookkeeping for the user agent.

Chrome's experimental implementation releases the tag after the event finishes for simplicity.