Fix Chrome crashes on calling OnTrayVisibilityChange

ash/common/system/status_area_widget.h

Index: ash/common/system/status_area_widget.h
diff --git a/ash/common/system/status_area_widget.h b/ash/common/system/status_area_widget.h
index bc74b20277ab332350f89629e2edea323cc69e36..f49d7b63bb0a5929e33e250ab64f7594028f3d69 100644
--- a/ash/common/system/status_area_widget.h
+++ b/ash/common/system/status_area_widget.h
@@ -27,6 +27,10 @@ class PaletteTray;
 class VirtualKeyboardTray;
 #endif
 
+namespace test {
+class SystemTrayTest;
+}
+
 class ASH_EXPORT StatusAreaWidget : public views::Widget,
                                     public ShelfBackgroundAnimatorObserver {
  public:
@@ -93,6 +97,8 @@ class ASH_EXPORT StatusAreaWidget : public views::Widget,
   void UpdateShelfItemBackground(int alpha) override;
 
  private:
+  friend class test::SystemTrayTest;
+
   void AddSystemTray();
   void AddWebNotificationTray();
 #if defined(OS_CHROMEOS)