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This CL adds MockBackgroundTaskCoordinator and create an interface for
BackgroundTaskCoordinator, also move implementation code to cc file.

Bug: 963304
Change-Id: I5308ca58d87d1e0c7e56b4b780955d132cc98c33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728346Reviewed-by: Hesen Zhang <hesen@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683240}