Do not use assets for Ash window control button backgrounds

ash/ash_layout_constants.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_ASH_LAYOUT_CONSTANTS_H_
+#define ASH_ASH_LAYOUT_CONSTANTS_H_
+
+#include "ash/ash_export.h"
+#include "ui/gfx/geometry/size.h"
+
+enum class AshLayoutSize {
+  // Size of a caption button in a maximized browser window.
+  BROWSER_MAXIMIZED_CAPTION_BUTTON,
+
+  // Size of a caption button in a restored browser window.
+  BROWSER_RESTORED_CAPTION_BUTTON,
+
+  // Size of a caption button in a non-browser window.
+  NON_BROWSER_CAPTION_BUTTON,
+};
+
+ASH_EXPORT gfx::Size GetAshLayoutSize(AshLayoutSize size);
+
+#endif  // ASH_ASH_LAYOUT_CONSTANTS_H_