Adds an ash global shortcut key for message center.

ash/system/web_notification/web_notification_tray.cc

Index: ash/system/web_notification/web_notification_tray.cc
diff --git a/ash/system/web_notification/web_notification_tray.cc b/ash/system/web_notification/web_notification_tray.cc
index 3db837c0ecd7c91ff4ec12866680ccf4c51e5191..7ea81aabd9159a6519b0b1980aa7303078efa568 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -249,6 +249,11 @@ bool WebNotificationTray::IsMouseInNotificationBubble() const {
       Shell::GetScreen()->GetCursorScreenPoint());
 }
 
+void WebNotificationTray::ShowMessageCenterBubble() {
+  if (!IsMessageCenterBubbleVisible())
+    message_center_tray_->ShowMessageCenterBubble();
+}
+
 void WebNotificationTray::SetShelfAlignment(ShelfAlignment alignment) {
   if (alignment == shelf_alignment())
     return;