Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/ui/libgtkui/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_
+#ifndef CHROME_BROWSER_UI_LIBGTKUI_NATIVE_THEME_GTK2_H_
+#define CHROME_BROWSER_UI_LIBGTKUI_NATIVE_THEME_GTK2_H_
 
 #include <gtk/gtk.h>
 
 #include "base/macros.h"
 #include "ui/native_theme/native_theme_base.h"
 
 
-namespace libgtk2ui {
+namespace libgtkui {
 
 // 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;
@@ skipping 23 matching lines @@
   GtkWidget* GetMenu() const;
   GtkWidget* GetMenuItem() const;
 
  private:
   NativeThemeGtk2();
   ~NativeThemeGtk2() override;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeGtk2);
 };
 
-}  // namespace libgtk2ui
+}  // namespace libgtkui
 
-#endif  // CHROME_BROWSER_UI_LIBGTK2UI_NATIVE_THEME_GTK2_H_
+#endif  // CHROME_BROWSER_UI_LIBGTKUI_NATIVE_THEME_GTK2_H_