[Background Fetch] Delete all in-memory fetch data

Inform upstream systems when a fetch is completely finished (event
dispatch is complete) to know that it's safe to remove in-memory
fetch data.

Bug: 904818
Change-Id: Ieef3ff87ce66e97f177ef8422994adb22568e155
Reviewed-on: https://chromium-review.googlesource.com/c/1333815
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608033}
13 files changed