views: don't use open-folder icons in TreeView on Mac

ui/views/style/platform_style.cc

Index: ui/views/style/platform_style.cc
diff --git a/ui/views/style/platform_style.cc b/ui/views/style/platform_style.cc
index 0dff5cb4ff7821916b28ffb56e08f73bdee153c9..ca8625589159254abcf270df7b282bcda00bef8b 100644
--- a/ui/views/style/platform_style.cc
+++ b/ui/views/style/platform_style.cc
@@ -49,6 +49,7 @@ const CustomButton::KeyClickAction PlatformStyle::kKeyClickActionOnSpace =
     CustomButton::CLICK_ON_KEY_RELEASE;
 const bool PlatformStyle::kReturnClicksFocusedControl = true;
 const bool PlatformStyle::kTreeViewSelectionPaintsEntireRow = false;
+const bool PlatformStyle::kTreeViewUsesOpenIcon = true;
 const bool PlatformStyle::kUseRipples = true;
 const bool PlatformStyle::kMirrorBubbleArrowInRTLByDefault = true;