{Profile}NotifierGroup had one purpose, which was to separate
notification settings from different profiles on Win-ash. ChromeOS
doesn't mix notification settings from different profiles and only shows
those from the active user, so they're not needed.

MessageCenterSettingsController is only used by NotifierSettingsView,
which now only compiles on ash/. Hence it can be ChromeOS only now
(although it's still difficult to move to ash/). This class gets a lot
simpler.

Other random cleanups:
- move more code into NotifierController
- cut NotificationUiManager out of the settings provider initialization
  loop; instead set it directly on MessageCenter.

Change-Id: Ib9a60e0eb3c167513eba555938369bad3f9d4634
Bug: 755413
Reviewed-on: https://chromium-review.googlesource.com/685986


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509987}