Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/ui/libgtk2ui/native_theme_gtk2.h

-// Copyright (c) 2013 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_LIBGTK2UI_NATIVE_THEME_GTK2_H_
-#define CHROME_BROWSER_UI_LIBGTK2UI_NATIVE_THEME_GTK2_H_
-
-#include <gtk/gtk.h>
-
-#include "base/macros.h"
-#include "ui/native_theme/native_theme_base.h"
-
-
-namespace libgtk2ui {
-
-// A version of NativeTheme that uses GTK2 supplied colours instead of the
-// default aura colours. Analogue to NativeThemeWin, except that can't be
-// compiled into the main chrome binary like the Windows code can.
-class NativeThemeGtk2 : public ui::NativeThemeBase {
- public:
-  static NativeThemeGtk2* instance();
-
-  // Overridden from ui::NativeThemeBase:
-  SkColor GetSystemColor(ColorId color_id) const override;
-  void PaintMenuPopupBackground(
-      SkCanvas* canvas,
-      const gfx::Size& size,
-      const MenuBackgroundExtraParams& menu_background) const override;
-  void PaintMenuItemBackground(
-      SkCanvas* canvas,
-      State state,
-      const gfx::Rect& rect,
-      const MenuItemExtraParams& menu_item) const override;
-
-  // Gets a ChromeGtkFrame theme color; returns true on success. Always returns
-  // false in GTK3.
-  bool GetChromeStyleColor(const char* style_property,
-                           SkColor* ret_color) const;
-
-  // Returns various widgets for theming use.
-  GtkWidget* GetWindow() const;
-  GtkWidget* GetEntry() const;
-  GtkWidget* GetLabel() const;
-  GtkWidget* GetButton() const;
-  GtkWidget* GetBlueButton() const;
-  GtkWidget* GetTree() const;
-  GtkWidget* GetTooltip() const;
-  GtkWidget* GetMenu() const;
-  GtkWidget* GetMenuItem() const;
-
- private:
-  NativeThemeGtk2();
-  ~NativeThemeGtk2() override;
-
-  DISALLOW_COPY_AND_ASSIGN(NativeThemeGtk2);
-};
-
-}  // namespace libgtk2ui
-
-#endif  // CHROME_BROWSER_UI_LIBGTK2UI_NATIVE_THEME_GTK2_H_