GitLab

For graceful shutdown, this CL introduces OnShutdown
observer callback.
Along with the change, EventRouter now properly calls RemoveObserver
in the callback.
ArcServiceManager::IsInitialized() is now no longer needed. Remove it.

BUG=672829
TEST=Ran bots.

Review-Url: https://codereview.chromium.org/2581953002
Cr-Commit-Position: refs/heads/master@{#439144}