[MD] Don't use new OTR native-theming when using a custom browser theme.

chrome/browser/ui/views/theme_copying_widget.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 CHROME_BROWSER_UI_VIEWS_THEME_COPYING_WIDGET_H_
+#define CHROME_BROWSER_UI_VIEWS_THEME_COPYING_WIDGET_H_
+
+#include "ui/views/widget/widget.h"
+
+// This widget uses a reference widget to provide its NativeTheme. The
+// reference widget is assumed to outlive |this|.
+class ThemeCopyingWidget : public views::Widget {
+ public:
+  explicit ThemeCopyingWidget(views::Widget* role_model);
+  ~ThemeCopyingWidget() override;
+
+  // views::Widget
+  const ui::NativeTheme* GetNativeTheme() const override;
+
+ private:
+  // The widget we'll copy our theme from.
+  views::Widget* role_model_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThemeCopyingWidget);
+};
+
+#endif  // CHROME_BROWSER_UI_VIEWS_THEME_COPYING_WIDGET_H_