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Previously the pref service was connected to the temporary login screen
profile prefs. Now it connects to the user profile on login and after
user switch.

Rename RegisterPrefs() to RegisterProfilePrefs() in ash to emphasize
that these are profile-associated and not local state.

Add ShellObserver notification of PrefService change. The lifetime is
tricky because the old PrefService must continue to exist so shell
observers can unregister their pref observers. Also, there is a window
using user switch where the user pref service is null.

This is based on https://chromium-review.googlesource.com/c/580115
and is a prerequisite for moving some system tray code out of chrome
into ash.

Bug: 747019, 721961, 736169
Test: added to ash_unittests
Change-Id: I186e55c1d88e0e2052b9cd76a490e26804e4b6d3
Reviewed-on: https://chromium-review.googlesource.com/590356
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490804}