Move knowledge about MessageCenter out of MessageView.

ui/message_center/views/message_center_view.h

Index: ui/message_center/views/message_center_view.h
diff --git a/ui/message_center/views/message_center_view.h b/ui/message_center/views/message_center_view.h
index e230c382bf8796045622d9bf31be77bec7ebefcd..10381b338144fa28015fcae6df7a63e4f9e6c163 100644
--- a/ui/message_center/views/message_center_view.h
+++ b/ui/message_center/views/message_center_view.h
@@ -31,6 +31,7 @@ class MessageCenterTray;
 class MessageCenterView;
 class MessageView;
 class MessageListView;
+class NotificationView;
 class NotifierSettingsView;
 
 // MessageCenterView ///////////////////////////////////////////////////////////
@@ -83,11 +84,14 @@ class MESSAGE_CENTER_EXPORT MessageCenterView : public views::View,
 
   void AddNotificationAt(const Notification& notification, int index);
   void NotificationsChanged();
-  void SetNotificationViewForTest(views::View* view);
+  void SetNotificationViewForTest(MessageView* view);
 
   MessageCenter* message_center_;  // Weak reference.
   MessageCenterTray* tray_;  // Weak reference.
-  std::vector<MessageView*> message_views_;  // Weak references.
+  // Map notification_id->NotificationView*. It contains all NotificaitonViews
+  // currently displayed in MessageCenter.
+  typedef std::map<std::string, NotificationView*> NotificationViewsMap;
+  NotificationViewsMap notification_views_;  // Weak.
 
   // Child views.
   views::ScrollView* scroller_;