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

ui/views/controls/combobox/combobox.h

Index: ui/views/controls/combobox/combobox.h
diff --git a/ui/views/controls/combobox/combobox.h b/ui/views/controls/combobox/combobox.h
index 12c0bec723f90cab0a7114a6be26018752872c00..ca039d79d2a93f7d6ccbc7e88dacbd986939e2cf 100644
--- a/ui/views/controls/combobox/combobox.h
+++ b/ui/views/controls/combobox/combobox.h
@@ -209,6 +209,9 @@ class VIEWS_EXPORT Combobox : public View,
   CustomButton* text_button_;
   CustomButton* arrow_button_;
 
+  // A child that indicates focus.
+  View* focus_ring_;
+
   // Set while the dropdown is showing. Ensures the menu is closed if |this| is
   // destroyed.
   std::unique_ptr<MenuModelAdapter> menu_model_adapter_;