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Currently, the ExtensionsSystem watches for application termination, and
then forwards the notification along to the apps code. This causes a
dependency between the extensions system and apps.

Move the application termination observation to a dedicated KeyedService
in platform_apps code, and remove knowledge of platform apps from
the extensions system.

Bug: 873872
Change-Id: I2dc6ae147acc4bad428542f5c5ed31154355304b
Reviewed-on: https://chromium-review.googlesource.com/1173765Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583158}