aura: Remove layerless windows.

ui/aura/window_layer_type.h

-// Copyright (c) 2012 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_AURA_WINDOW_LAYER_TYPE_H_
-#define UI_AURA_WINDOW_LAYER_TYPE_H_
-
-namespace aura {
-
-// These constants mirror that of ui::LayerType with the addition of
-// WINDOW_LAYER_NONE. See ui::LayerType for description of ones in common.
-enum WindowLayerType {
-  // Note that Windows with WINDOW_LAYER_NONE impose limitations on the
-  // Window: transforms and animations aren't supported.
-  WINDOW_LAYER_NONE,
-
-  WINDOW_LAYER_NOT_DRAWN,
-  WINDOW_LAYER_TEXTURED,
-  WINDOW_LAYER_SOLID_COLOR,
-};
-
-}  // namespace aura
-
-#endif  // UI_AURA_WINDOW_LAYER_TYPE_H_