Makes mouse-hover prevent notification toasts from moving.

ui/message_center/views/message_popup_collection.h

Index: ui/message_center/views/message_popup_collection.h
diff --git a/ui/message_center/views/message_popup_collection.h b/ui/message_center/views/message_popup_collection.h
index 0d3d5075a3684da150a610923a123960f24bb8e8..e0af09c03832408ff28b7aeb8e3987a8cba35104 100644
--- a/ui/message_center/views/message_popup_collection.h
+++ b/ui/message_center/views/message_popup_collection.h
@@ -58,8 +58,10 @@ class MESSAGE_CENTER_EXPORT MessagePopupCollection
   // Called by ToastContentsView when its window is closed.
   void RemoveToast(ToastContentsView* toast);
 
-  void OnMouseEntered();
-  void OnMouseExited();
+  // Since these events are really coming from individual toast widgets,
+  // it helps to be able to keep track of the sender.
+  void OnMouseEntered(ToastContentsView* toast_entered);
+  void OnMouseExited(ToastContentsView* toast_exited);
 
   // Invoked by toasts when they start/finish their animations.
   // While "defer counter" is greater then zero, the popup collection does
@@ -129,6 +131,10 @@ class MESSAGE_CENTER_EXPORT MessagePopupCollection
 
   int defer_counter_;
 
+  // This is only used to compare with incoming events, do not assume that
+  // the toast will be valid if this pointer is non-NULL.
+  ToastContentsView* latest_toast_entered_;
+
   // Denotes a mode when user is clicking the Close button of toasts in a
   // sequence, w/o moving the mouse. We reposition the toasts so the next one
   // happens to be right under the mouse, and the user can just dispose of