BoxLayout now suports per-view margins.

ui/views/view_properties.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_VIEWS_VIEW_PROPERTIES_H_
+#define UI_VIEWS_VIEW_PROPERTIES_H_
+
+#include "ui/base/class_property.h"
+#include "ui/gfx/geometry/insets.h"
+#include "ui/views/views_export.h"
+
+namespace views {
+
+// A property to store margins around the outer perimeter of the view. Margins
+// are outside the bounds of the view. This is used by various layout managers
+// to position views with the proper spacing between them.
+VIEWS_EXPORT extern const ui::ClassProperty<gfx::Insets*>* const kMarginsKey;
+
+}  // namespace views
+
+#endif  // UI_VIEWS_VIEW_PROPERTIES_H_