Refactor MdFocusRing to make it easier to reuse on other controls.

ui/views/controls/textfield/textfield.h

Index: ui/views/controls/textfield/textfield.h
diff --git a/ui/views/controls/textfield/textfield.h b/ui/views/controls/textfield/textfield.h
index c6dd5c11eaa2def01c015a62a224062858bb2549..40e5b17b746675fd22cc1ee6f9957ff6ffa71b5b 100644
--- a/ui/views/controls/textfield/textfield.h
+++ b/ui/views/controls/textfield/textfield.h
@@ -508,6 +508,9 @@ class VIEWS_EXPORT Textfield : public View,
   std::unique_ptr<ui::SimpleMenuModel> context_menu_contents_;
   std::unique_ptr<views::MenuRunner> context_menu_runner_;
 
+  // A child that indicates focus.
+  View* focus_ring_;
+
   // Used to bind callback functions to this object.
   base::WeakPtrFactory<Textfield> weak_ptr_factory_;