Add context to gfx::Screen calls in support of simultaneous desktop+ash

ash/system/tray/tray_background_view.h

Index: ash/system/tray/tray_background_view.h
diff --git a/ash/system/tray/tray_background_view.h b/ash/system/tray/tray_background_view.h
index 9f33db7d212937925d25c101cdfed7e984ffc5d7..9b4b3db6ec97046983a9f1660fa1a93d6d4890ca 100644
--- a/ash/system/tray/tray_background_view.h
+++ b/ash/system/tray/tray_background_view.h
@@ -99,6 +99,9 @@ class ASH_EXPORT TrayBackgroundView : public internal::ActionableView,
   StatusAreaWidget* status_area_widget() {
     return status_area_widget_;
   }
+  const StatusAreaWidget* status_area_widget() const {
+    return status_area_widget_;
+  }
   TrayContainer* tray_container() const { return tray_container_; }
   ShelfAlignment shelf_alignment() const { return shelf_alignment_; }