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 1466c293cd9b288c0a146a4d13b4ed0bfc97de8e..8a1a16533f9afe50b781096a637d6b5c75018070 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -63,6 +63,9 @@ class ASH_EXPORT WebNotificationTray
   // Returns true if the mouse is inside the notification bubble.
   bool IsMouseInNotificationBubble() const;
 
+  // Shows the message center bubble.
+  void ShowMessageCenterBubble();
+
   // Overridden from TrayBackgroundView.
   virtual void SetShelfAlignment(ShelfAlignment alignment) OVERRIDE;
   virtual void AnchorUpdated() OVERRIDE;
@@ -119,7 +122,6 @@ class ASH_EXPORT WebNotificationTray
   // Shows the quiet mode bubble.
   void ShowQuietModeBubble();
 
-
   internal::WebNotificationBubbleWrapper* message_center_bubble() const {
     return message_center_bubble_.get();
   }