Add a test of updating cached height for popup notifications

ash/system/web_notification/ash_popup_alignment_delegate.h

Index: ash/system/web_notification/ash_popup_alignment_delegate.h
diff --git a/ash/system/web_notification/ash_popup_alignment_delegate.h b/ash/system/web_notification/ash_popup_alignment_delegate.h
index 50b70621a5748d17eea727bdbdb7b0e911017fec..54a8e284915f9b8820bd1203d81cac7c02411349 100644
--- a/ash/system/web_notification/ash_popup_alignment_delegate.h
+++ b/ash/system/web_notification/ash_popup_alignment_delegate.h
@@ -48,6 +48,9 @@ class ASH_EXPORT AshPopupAlignmentDelegate
   // can avoid it.
   void SetSystemTrayHeight(int height);
 
+  // Returns the current system tray height.
+  int system_tray_height_for_test() const { return system_tray_height_; }
+
   // Overridden from message_center::PopupAlignmentDelegate:
   int GetToastOriginX(const gfx::Rect& toast_bounds) const override;
   int GetBaseLine() const override;