Start delegating notifications to MessageCenter on Windows.

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 05e6f6e7ba5aa7defaf5582d8afb58cc69795d9e..26c5874379ac5954af25b61d0435323b5a128a51 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -83,7 +83,7 @@ WebNotificationTray::WebNotificationTray(
     : internal::TrayBackgroundView(status_area_widget),
       button_(NULL),
       show_message_center_on_unlock_(false) {
-  message_center_ = message_center::MessageCenter::GetInstance();
+  message_center_ = ash::Shell::GetInstance()->message_center();
   message_center_->AddObserver(this);
   button_ = new views::ImageButton(this);
   button_->set_triggerable_event_flags(
@@ -94,7 +94,7 @@ WebNotificationTray::WebNotificationTray(
 }
 
 WebNotificationTray::~WebNotificationTray() {
-  // Ensure the message center doesn't notify a destroyed object.
+  // Ensure the message center doesn't notify an object under destruction.
   message_center_->RemoveObserver(this);
   // Release any child views that might have back pointers before ~View().
   message_center_bubble_.reset();
@@ -118,7 +118,7 @@ void WebNotificationTray::ShowMessageCenterBubble() {
   UpdateTray();
   HidePopupBubble();
   message_center::MessageCenterBubble* bubble =
-      new message_center::MessageCenterBubble(message_center_);
+      new message_center::MessageCenterBubble(message_center_.get());
   // Sets the maximum height of the bubble based on the screen.
   // TODO(mukai): move this to WebNotificationBubbleWrapper if it's safe
   // to set the height of the popup.
@@ -172,7 +172,7 @@ void WebNotificationTray::ShowPopupBubble() {
     popup_bubble_.reset(
         new internal::WebNotificationBubbleWrapper(
             this, new message_center::MessagePopupBubble(
-                message_center_)));
+                message_center_.get())));
   }
 }