[Background Fetch] Keep Job Details in memory until safe to erase

The Job Details were being deleted in the delegate after the event was
dispatched, which broke the click event. This change keeps the details
around until after the last click event, since currently there is no way
to figure out whether the notification still exists or not.

Change-Id: Ia43534b763d5039ee8bffa153899aee4b706132c
Reviewed-on: https://chromium-review.googlesource.com/c/1401063
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@google.com>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621724}
3 files changed