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The CL adds a general-purpose feature flag, and creates
DevToolsBackgroundServicesContext which is part of the StoragePartition.

The context will be responsible for logging the events from the relevant
features (BackgroundFetch/BackgroundSync), and will be accessible from
the soon-to-be-added DevTools protocol handler (via the StoragePartition)
to perform the reads/writes.

Bug: 927726

Change-Id: I7c48a1bac742b9e6e8eaed52f8e69eace6c25f1b
Reviewed-on: https://chromium-review.googlesource.com/c/1407077
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630067}