Reland: Show message center on lock screen

ui/message_center/message_center_observer.h

Index: ui/message_center/message_center_observer.h
diff --git a/ui/message_center/message_center_observer.h b/ui/message_center/message_center_observer.h
index 5ab0722be13c1e9323085dcd8bf4c6743837c74f..e6315ccb9299267ce08d0f8ca13dfd854b3b37c9 100644
--- a/ui/message_center/message_center_observer.h
+++ b/ui/message_center/message_center_observer.h
@@ -56,6 +56,9 @@ class MESSAGE_CENTER_EXPORT MessageCenterObserver {
   // quiet mode expires.
   virtual void OnQuietModeChanged(bool in_quiet_mode) {}
 
+  // Called when the user locks (or unlocks) the screen.
+  virtual void OnLockedStateChanged(bool locked) {}
+
   // Called when the blocking state of |blocker| is changed.
   virtual void OnBlockingStateChanged(NotificationBlocker* blocker) {}
 };