Make MessageCenter a singleton object and build 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 5e7822fbb8d3c063863e94884f5916ff799a8a78..4c8d839ebb3d907b4f92c2c1123b6993ef47fd4a 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -83,7 +83,8 @@ WebNotificationTray::WebNotificationTray(
     : internal::TrayBackgroundView(status_area_widget),
       button_(NULL),
       show_message_center_on_unlock_(false) {
-  message_center_.reset(new message_center::MessageCenter(this));
+  message_center_ = message_center::MessageCenter::GetInstance();
+  message_center_->SetHost(this);
   button_ = new views::ImageButton(this);
   button_->set_triggerable_event_flags(
       ui::EF_LEFT_MOUSE_BUTTON | ui::EF_RIGHT_MOUSE_BUTTON);
@@ -93,7 +94,7 @@ WebNotificationTray::WebNotificationTray(
 
 WebNotificationTray::~WebNotificationTray() {
   // message_center_ has a weak pointer to this; destroy it early.
-  message_center_.reset();
+  message_center_->SetHost(NULL);
   // Release any child views that might have back pointers before ~View().
   message_center_bubble_.reset();
   popup_bubble_.reset();
@@ -116,7 +117,7 @@ void WebNotificationTray::ShowMessageCenterBubble() {
   UpdateTray();
   HidePopupBubble();
   message_center::MessageCenterBubble* bubble =
-      new message_center::MessageCenterBubble(message_center_.get());
+      new message_center::MessageCenterBubble(message_center_);
   message_center_bubble_.reset(
       new internal::WebNotificationBubbleWrapper(this, bubble));
 
@@ -156,7 +157,7 @@ void WebNotificationTray::ShowPopupBubble() {
     popup_bubble_.reset(
         new internal::WebNotificationBubbleWrapper(
             this, new message_center::MessagePopupBubble(
-                message_center_.get())));
+                message_center_)));
   }
 }