Make gtk2/gtk3 compile time switchable.

build/config/ui.gni

 # Copyright 2014 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.
 
 # This file contains UI-related build flags (see features.gni for Chrome
 # feature-related ones). These should theoretically be moved to the ui
 # directory.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
 # e.g. base, so they need to be global to match the GYP configuration. Also,
@@ skipping 17 matching lines @@
 
   # Indicates if Aura is enabled. Aura is a low-level windowing library, sort
   # of a replacement for GDI or GTK.
   use_aura = is_win || is_linux
 
   # True means the UI is built using the "views" framework.
   toolkit_views = is_mac || is_win || is_chromeos || use_aura
 
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
+
+  # Whether we should use GTKv3 instead of GTKv2.
+  use_gtk3 = false
 }
 
 # Additional dependent variables -----------------------------------------------
 #
 # These variables depend on other variables and can't be set externally.
 
 # Use GPU accelerated cross process image transport by default on linux builds
 # with the Aura window manager.
 ui_compositor_image_transport = use_aura && is_linux
 
@@ skipping 16 matching lines @@
 # Whether to use atk, the Accessibility ToolKit library
 use_atk = is_desktop_linux && use_x11
 
 use_ozone_evdev = use_ozone
 
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
 enable_hidpi = is_mac || is_win || is_linux
 
 enable_topchrome_md = is_chromeos || is_win || is_linux