Don't call AnimateClearingOneNotification if clearing all is already started.

ui/message_center/views/message_list_view.cc

Index: ui/message_center/views/message_list_view.cc
diff --git a/ui/message_center/views/message_list_view.cc b/ui/message_center/views/message_list_view.cc
index 2551fab85d2a92830994aee3b0e5e6bf693a9000..bd731b70ee0047419d31b7e9bcce308a5658cc07 100644
--- a/ui/message_center/views/message_list_view.cc
+++ b/ui/message_center/views/message_list_view.cc
@@ -312,7 +312,8 @@ void MessageListView::DoUpdateIfPossible() {
   }
 
   if (!clearing_all_views_.empty()) {
-    AnimateClearingOneNotification();
+    if (!clear_all_started_)
+      AnimateClearingOneNotification();
     return;
   }