views: don't use open-folder icons in TreeView 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 5e31fa2f66406dd9fa5a04d9576fa177cff569c8..876dba51c3a2e52fc117278912606b9740c6ec75 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -60,6 +60,10 @@ class VIEWS_EXPORT PlatformStyle {
   // label for that row.
   static const bool kTreeViewSelectionPaintsEntireRow;
 
+  // Whether TreeViews use a separate icon for the currently selected node's
+  // ancestors.
+  static const bool kTreeViewUsesOpenIcon;
+
   // Whether ripples should be used for visual feedback on control activation.
   static const bool kUseRipples;