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 e8c9e70bc73f43388104dcc6546d6b0ba0f2b15a..bac13fe3b3cdf8e6aedaf4dab2633cf0fc68f9f0 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -286,6 +286,10 @@ void WebNotificationTray::SetSystemTrayHeight(int height) {
   popup_alignment_delegate_->SetSystemTrayHeight(height);
 }
 
+int WebNotificationTray::system_tray_height() const {
+  return popup_alignment_delegate_->system_tray_height();
+}
+
 bool WebNotificationTray::ShowPopups() {
   if (message_center_bubble())
     return false;