Revert of 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 e6315ccb9299267ce08d0f8ca13dfd854b3b37c9..5ab0722be13c1e9323085dcd8bf4c6743837c74f 100644
--- a/ui/message_center/message_center_observer.h
+++ b/ui/message_center/message_center_observer.h
@@ -56,9 +56,6 @@
   // 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) {}
 };