Re-factor Ash Message Center code part 4/4

ash/system/web_notification/web_notification_list.h

Index: ash/system/web_notification/web_notification_list.h
diff --git a/ash/system/web_notification/web_notification_list.h b/ash/system/web_notification/web_notification_list.h
index 4fb241610783330f4105bf85f0d75b2064dc7ec4..74d06f420bc57a69d97001a1c127a5d3b6f8b979 100644
--- a/ash/system/web_notification/web_notification_list.h
+++ b/ash/system/web_notification/web_notification_list.h
@@ -44,6 +44,7 @@ class ASH_EXPORT WebNotificationList {
   explicit WebNotificationList(Delegate* delegate);
   virtual ~WebNotificationList();
 
+  // Affects whether or not a message has been "read".
   void SetMessageCenterVisible(bool visible);
 
   void AddNotification(const std::string& id,
@@ -84,7 +85,7 @@ class ASH_EXPORT WebNotificationList {
   void MarkPopupsAsShown();
 
   const Notifications& notifications() const { return notifications_; }
-  int unread_count() const { return unread_count_; }
+  size_t unread_count() const { return unread_count_; }
 
   static const size_t kMaxVisiblePopupNotifications;
   static const size_t kMaxVisibleMessageCenterNotifications;
@@ -106,7 +107,7 @@ class ASH_EXPORT WebNotificationList {
   Delegate* delegate_;
   Notifications notifications_;
   bool message_center_visible_;
-  int unread_count_;
+  size_t unread_count_;
 
   DISALLOW_COPY_AND_ASSIGN(WebNotificationList);
 };