Sends the scroll gesture to scrollbar.

ui/message_center/message_view.cc

Index: ui/message_center/message_view.cc
diff --git a/ui/message_center/message_view.cc b/ui/message_center/message_view.cc
index 7618ec05ecb6cb8bf00458341dbb3de38ccd1ac0..56d3a3b46a4d15d9cb92e2b20c258731763a2ad0 100644
--- a/ui/message_center/message_view.cc
+++ b/ui/message_center/message_view.cc
@@ -264,8 +264,9 @@ void MessageView::OnGestureEvent(ui::GestureEvent* event) {
   }
 
   SlideOutView::OnGestureEvent(event);
-  if (event->handled())
-    return;
+  // Do not return here by checking handled(). SlideOutView calls SetHandled()
+  // even though the scroll gesture doesn't make no (or little) effects on the
+  // slide-out behavior. See http://crbug.com/172991
 
   if (!event->IsScrollGestureEvent())
     return;