Harmony - Use FocusableBorder for comboboxes on Mac

ui/views/style/platform_style.h

Index: ui/views/style/platform_style.h
diff --git a/ui/views/style/platform_style.h b/ui/views/style/platform_style.h
index 8405cb2009a6d2284b297e80343f7efd728c9cde..c96f726ce7bc4059d61c6be98d56976d69581e2f 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -65,10 +65,6 @@ class VIEWS_EXPORT PlatformStyle {
   // Creates the appropriate border for a focusable Combobox.
   static std::unique_ptr<FocusableBorder> CreateComboboxBorder();
 
-  // Creates the appropriate background for a Combobox.
-  static std::unique_ptr<Background> CreateComboboxBackground(
-      int shoulder_width);
-
   // Creates the default scrollbar for the given orientation.
   static std::unique_ptr<ScrollBar> CreateScrollBar(bool is_horizontal);