Use fetch_event_id (fetchEventID in blink) for FetchEvent
Currently we have |event_finish_id| in both browser and renderer code, but they
aren't the same thing. To avoid this confusion, this CL will use |fetch_event_id|
(fetchEventID in blink) for FetchEvent handling. And will stop using
|event_finish_id| in renderer code by using AddWithID() in
ServiceWorkerContextClient::DispatchFetchEvent().
BUG=
649558
Committed:
https://crrev.com/160d7098f1c9f293990f0a1ee6644442ad945fc9
Cr-Commit-Position: refs/heads/master@{#425255}