Make gtk2/gtk3 compile time switchable.

chrome/browser/ui/libgtk2ui/libgtk2ui.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       # GN version: //chrome/browser/ui/libgtk2ui
       'target_name': 'gtk2ui',
       'type': '<(component)',
       'dependencies': [
         '../../../../base/base.gyp:base',
         '../../../../base/base.gyp:base_i18n',
         '../../../../base/third_party/dynamic_annotations/dynamic_annotations.gyp:dynamic_annotations',
         '../../../../build/linux/system.gyp:gconf',
-        '../../../../build/linux/system.gyp:gtk',
-        '../../../../build/linux/system.gyp:gtkprint',
         '../../../../build/linux/system.gyp:x11',
         '../../../../components/components_resources.gyp:components_resources',
         '../../../../content/content.gyp:content',
         '../../../../printing/printing.gyp:cups',
         '../../../../printing/printing.gyp:printing',
         '../../../../skia/skia.gyp:skia',
         '../../../../ui/aura/aura.gyp:aura',
         '../../../../ui/base/ime/ui_base_ime.gyp:ui_base_ime',
         '../../../../ui/base/ui_base.gyp:ui_base',
         '../../../../ui/events/events.gyp:events',
@@ skipping 68 matching lines @@
         'x11_input_method_context_impl_gtk2.cc',
         'x11_input_method_context_impl_gtk2.h',
       ],
       'conditions': [
         ['use_gconf==0', {
           'sources!': [
             'gconf_listener.cc',
             'gconf_listener.h',
           ],
         }],
+        ['use_gtk3==1', {
+          'dependencies': [
+            '../../../../build/linux/system.gyp:gtk3',
+            '../../../../build/linux/system.gyp:gtkprint3',
+          ],
+        }, {
+          'dependencies': [
+            '../../../../build/linux/system.gyp:gtk2',
+            '../../../../build/linux/system.gyp:gtkprint2',
+          ],
+        }],
         [ 'clang==1', {
           'cflags': [
             # G_DEFINE_TYPE automatically generates a *get_instance_private inline function after glib 2.37.
             # That's unused. Prevent to complain about it.
             '-Wno-unused-function',
 
             # G_STATIC_ASSERT uses a typedef as a static_assert.
             '-Wno-unused-local-typedef',
           ],
         }],
       ],
     },
   ],
 }