Add a test of updating cached height for popup notifications

ash/system/web_notification/web_notification_tray.cc

Index: ash/system/web_notification/web_notification_tray.cc
diff --git a/ash/system/web_notification/web_notification_tray.cc b/ash/system/web_notification/web_notification_tray.cc
index a23bf8965d0c7ca4ec813ad1ff28220197bd6a17..24912a4a67dd1be39cb4c05708c41d46249a7a83 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -285,6 +285,10 @@ void WebNotificationTray::SetSystemTrayHeight(int height) {
   popup_alignment_delegate_->SetSystemTrayHeight(height);
 }
 
+int WebNotificationTray::system_tray_height_for_test() const {
+  return popup_alignment_delegate_->system_tray_height_for_test();
+}
+
 bool WebNotificationTray::ShowPopups() {
   if (message_center_bubble())
     return false;