Adds an ash global shortcut key for message center.

ash/system/web_notification/web_notification_tray.h

Index: ash/system/web_notification/web_notification_tray.h
diff --git a/ash/system/web_notification/web_notification_tray.h b/ash/system/web_notification/web_notification_tray.h
index bd9e026af42db40d4f0ee85a98afbf0ecfd4d8ee..eed4678219c2a5164790061014bd007d7a38507e 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -77,6 +77,9 @@ class ASH_EXPORT WebNotificationTray
     return message_center_;
   }
 
+  // Shows or hides the message center bubble.
+  void ToggleMessageCenterBubble();
+
   // Overridden from TrayBackgroundView.
   virtual void SetShelfAlignment(ShelfAlignment alignment) OVERRIDE;
   virtual void AnchorUpdated() OVERRIDE;
@@ -115,9 +118,6 @@ class ASH_EXPORT WebNotificationTray
                            ManyMessageCenterNotifications);
   FRIEND_TEST_ALL_PREFIXES(WebNotificationTrayTest, ManyPopupNotifications);
 
-  // Shows or hides the message center bubble.
-  void ToggleMessageCenterBubble();
-
   // Shows or updates the message center bubble and hides the popup bubble.
   void ShowMessageCenterBubble();