Hide notifications popups when entering fullscreen mode.

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 205d84e3dcc969a8cbd9c34d4e2c24b8718a20aa..c01aae646a3d2fb5548525b8f47910b2a78600d5 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -427,6 +427,10 @@ bool WebNotificationTray::ShowNotifierSettings() {
   return ShowMessageCenterInternal(true /* show_settings */);
 }
 
+message_center::MessageCenterTray* WebNotificationTray::GetMessageCenterTray() {
+  return message_center_tray_.get();
+}
+
 bool WebNotificationTray::IsPressed() {
   return IsMessageCenterBubbleVisible();
 }