[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/native_theme/native_theme_dark_aura.h

+// Copyright 2015 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_NATIVE_THEME_NATIVE_THEME_DARK_AURA_H_
+#define UI_NATIVE_THEME_NATIVE_THEME_DARK_AURA_H_
+
+#include "base/macros.h"
+#include "ui/native_theme/native_theme_aura.h"
+
+namespace ui {
+
+// Aura implementation of native theme support for dark mode (used for
+// incognito).
+class NATIVE_THEME_EXPORT NativeThemeDarkAura : public NativeThemeAura {
+ public:
+  static NativeThemeDarkAura* instance();
+
+  // Overridden from NativeThemeBase:
+  SkColor GetSystemColor(ColorId color_id) const override;
+
+ private:
+  NativeThemeDarkAura();
+  ~NativeThemeDarkAura() override;
+
+  DISALLOW_COPY_AND_ASSIGN(NativeThemeDarkAura);
+};
+
+}  // namespace ui
+
+#endif  // UI_NATIVE_THEME_NATIVE_THEME_DARK_AURA_H_