Moves WmWindow::GetInternalWidget() into a standalone function

ash/wm/widget_finder.cc

+// 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.
+
+#include "ash/wm/widget_finder.h"
+
+#include "ash/wm/window_properties.h"
+#include "ui/aura/window.h"
+#include "ui/views/widget/widget.h"
+
+namespace ash {
+
+views::Widget* GetInternalWidgetForWindow(aura::Window* window) {
+  return window->GetProperty(kWidgetCreationTypeKey) ==
+                 WidgetCreationType::INTERNAL
+             ? views::Widget::GetWidgetForNativeView(window)
+             : nullptr;
+}
+
+}  // namespace ash