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Currently the notification entry is inserted into stl container after
loading the icon, which causes the background task coordinator failed
to find the notification and won't trigger any background task.

This CL inserts the entry when all asynchronous opeartions are done,
and reply the result in a callback.

This behavior should be covered with end to end test.

Bug: 998998
Change-Id: I00d9f3b8c3f6b93c37fa8d93dc319e0c1aa91f11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779567Reviewed-by: Hesen Zhang <hesen@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692846}