Implements the dialog view for logout button tray in public sessions.

ash/system/logout_button/logout_button_observer.h

Index: ash/system/logout_button/logout_button_observer.h
diff --git a/ash/system/logout_button/logout_button_observer.h b/ash/system/logout_button/logout_button_observer.h
index 9234c7d758d9c80b13ab2526543ddc2a035655a1..96964623d6b36fafe870eca97b06d0080e15431e 100644
--- a/ash/system/logout_button/logout_button_observer.h
+++ b/ash/system/logout_button/logout_button_observer.h
@@ -7,14 +7,20 @@
 
 namespace ash {
 
-// Observer for the value of the kShowLogoutButtonInTray pref that determines
-// whether a logout button should be shown in the system tray during a session.
+// Observer for the values of the kShowLogoutButtonInTray pref and
+// kLogoutDialogDurationMs prefs. kShowLogoutButtonInTray determines whether a
+// logout button should be shown in the system tray during a session.
+// kLogoutDialogDurationMs is the duration in milliseconds of the logout
+// confirmation dialog after user pressed logout button.
 class LogoutButtonObserver {
  public:
   virtual ~LogoutButtonObserver() {}
 
   // Called when the value of the kShowLogoutButtonInTray pref changes.
   virtual void OnShowLogoutButtonInTrayChanged(bool show) = 0;
+
+  // Called when the value of the kLogoutDialogDurationMs pref changes.
+  virtual void OnLogoutDialogDurationChanged(int duration) = 0;

[bartfab (slow), 2013/10/30 11:17:28]

I think the conversion should happen at a lower level. This is an interface for
passing configuration settings to the power button. The fact that these settings
come from prefs is an implementation detail. The settings should be read from
their source (a pref in this case), converted to the most appropriate data
format (base::TimeDelta in this case) and then sent to the button.

[binjin, 2013/11/19 14:43:46]

Done.

 };
 
 }  // namespace ash