Add DeviceSettingsService.

chrome/common/chrome_notification_types.h

Index: chrome/common/chrome_notification_types.h
diff --git a/chrome/common/chrome_notification_types.h b/chrome/common/chrome_notification_types.h
index bbfc58c85f5d6f9ef0bf89b429c560a06466554e..a37b1cb2b96b1895abb6eac39a413dce8b9d82bb 100644
--- a/chrome/common/chrome_notification_types.h
+++ b/chrome/common/chrome_notification_types.h
@@ -1070,6 +1070,12 @@ enum NotificationType {
   // Sent after UserManager checked ownership status of logged in user.
   NOTIFICATION_OWNERSHIP_CHECKED,
 
+  // Sent by DeviceSettingsService to indicate that the ownership status
+  // changed. If you can, please use DeviceSettingsService::Observer instead.
+  // Other singleton-based services can't use that because Observer
+  // unregistration is impossible due to unpredictable deletion order.
+  NOTIFICATION_OWNERSHIP_STATUS_CHANGED,

[Ben Goodger (Google), 2012/08/27 15:44:09]

General comment for whoever works on this stuff - does it need to use
notifications?

Notifications in general should be a last resort.

[Mattias Nissler (ping if slow), 2012/08/27 16:23:46]

I agree wholeheartedly :)

I've tried to capture the point to avoid notifications in the comment I put on
this particular one. Note that the three that come before this one I'm removing
in a follow-up CL here: http://codereview.chromium.org/10823040/

IMHO, a general issue with Chrome OS code in the browser is that relies too
heavily on singletons. That means there is no well-defined
initialization/shutdown order, thus making observer deregistration impossible
when one of the singletons needs to listen for a signal from another singleton.
Even worse, there are cycles in the listens-for-signal-from graph... don't get
me started :)

I'm trying to clean up the code I find while I'm at it, but cutting down on the
number of singletons is certainly a larger effort.

+
   // This is sent to a ChromeOS settings observer when a system setting is
   // changed. The source is the CrosSettings and the details a std::string of
   // the changed setting.