Add a test of updating cached height for popup notifications

ash/system/web_notification/web_notification_tray.h

Index: ash/system/web_notification/web_notification_tray.h
diff --git a/ash/system/web_notification/web_notification_tray.h b/ash/system/web_notification/web_notification_tray.h
index dd02ad6420a20f29e5cd386e01abdbf626c0092b..e5b8173a9f4eb8c81b990e20da046dec71ff1bd1 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -61,6 +61,9 @@ class ASH_EXPORT WebNotificationTray
   // shown in the system tray side.
   void SetSystemTrayHeight(int height);
 
+  // Returns the current system tray height.
+  int system_tray_height_for_test() const;
+
   // Returns true if it should block the auto hide behavior of the shelf.
   bool ShouldBlockShelfAutoHide() const;