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SessionManager is released in PostMainMessageLoopRun that
happens before PostDestroyThreads. ml::UserActivityManager
is released with DbusServices PostDestroyThreads. Hence it
crashes on shutdown when trying to un-observe SessionManager.

This CL adds a OnSessionManagerDestroyed to SessionManagerObserver
interface so that ml::UserActivityManager could clean up in time.

Bug: 987010
Change-Id: I05043a3e0b946bfb9e733ad476dacc35e9a9a3b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715401Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680197}