Moves WmWindow::GetInternalWidget() into a standalone function

ash/wm/widget_finder.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 ASH_WIDGET_FINDER_H_
+#define ASH_WIDGET_FINDER_H_
+
+#include "ash/ash_export.h"
+
+namespace aura {
+class Window;
+}
+
+namespace views {
+class Widget;
+}
+
+namespace ash {
+
+// Returns the widget associated with |window|, or null if not associated with
+// a widget. Only ash system UI widgets are returned, not widgets created
+// by the mus window manager code to show a non-client frame.
+ASH_EXPORT views::Widget* GetInternalWidgetForWindow(aura::Window* window);
+
+}  // namespace ash
+
+#endif  // ASH_WIDGET_FINDER_H_