GitLab

This CL rewrites the logic to schedule background task in
BackgroundTaskCoordinator. Now it's based on deliver time window.

The old unit tests are deleted, new unit tests will be added in
following CL.

Bug: 991010
Change-Id: I8a05e8178d94b1daadf87d8abc8f9e0bac783e37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756439
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689105}