Converts overview to use common ash types

ash/wm/common/wm_window_property.h

 // Copyright 2016 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 ASH_WM_COMMON_WM_WINDOW_PROPERTY_H_
 #define ASH_WM_COMMON_WM_WINDOW_PROPERTY_H_
 
 namespace ash {
 namespace wm {
 
 enum class WmWindowProperty {
   // Type bool.
   SNAP_CHILDREN_TO_PIXEL_BOUDARY,
 
   // Type bool.
   ALWAYS_ON_TOP,
 
   // Type int.
   SHELF_ID,
+
+  // Type int. See aura::client::kTopViewInset for details.
+  TOP_VIEW_INSET,
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_COMMON_WM_WINDOW_PROPERTY_H_