Make gtk2/gtk3 compile time switchable.

chrome/browser/ui/libgtk2ui/BUILD.gn

 # 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.
 
 assert(is_linux, "This file should only be referenced on Linux")
 
 import("//build/config/features.gni")
+import("//build/config/ui.gni")
 
 component("libgtk2ui") {
   sources = [
     "app_indicator_icon.cc",
     "app_indicator_icon.h",
     "app_indicator_icon_menu.cc",
     "app_indicator_icon_menu.h",
     "chrome_gtk_frame.cc",
     "chrome_gtk_frame.h",
     "chrome_gtk_menu_subclasses.cc",
@@ skipping 38 matching lines @@
   ]
 
   if (use_gconf) {
     sources += [
       "gconf_listener.cc",
       "gconf_listener.h",
     ]
   }
   defines = [ "LIBGTK2UI_IMPLEMENTATION" ]
 
-  configs += [
-    "//build/config/linux:gconf",
-  ]
+  configs += [ "//build/config/linux:gconf" ]
 
   if (use_cups) {
-    configs += [
-      "//printing:cups",
-    ]
+    configs += [ "//printing:cups" ]
   }
 
   # GTK2 pulls pangoft2 as dependency, and pangoft2 depends on harfbuzz.
   # To avoid missing indirectly referenced harfbuzz symbols from pango,
   # some hack is required when bundled harfbuzz is used and component build is
   # disabled.
   # See crbug.com/462689 for details.
   all_dependent_configs = [ "//third_party/harfbuzz-ng:pangoft2_link_hack" ]
 
   # gn orders flags on a target before flags from configs. The default config
@@ skipping 13 matching lines @@
   }
   configs += [
     ":libgtk2ui_warnings",
     "//build/config/linux:x11",
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
     "//base:i18n",
-    "//build/config/linux/gtk",
-    "//build/config/linux/gtk:gtkprint",
     "//chrome/app/theme:theme_resources",
     "//chrome:extra_resources",
     "//chrome:resources",
     "//chrome:strings",
     "//components/resources",
     "//content/public/browser",
     "//mojo/environment:chromium",
     "//printing",
     "//skia",
     "//third_party/mojo/src/mojo/edk/system",
     "//ui/aura",
     "//ui/base",
     "//ui/base/ime",
     "//ui/events",
     "//ui/events:events_base",
     "//ui/gfx",
     "//ui/gfx/x",
     "//ui/native_theme",
     "//ui/resources",
     "//ui/shell_dialogs",
     "//ui/strings",
     "//ui/views",
   ]
+
+  if (use_gtk3) {
+    deps += [
+      "//build/config/linux/gtk3",
+      "//build/config/linux/gtk3:gtkprint3",
+    ]
+  } else {
+    deps += [
+      "//build/config/linux/gtk2",
+      "//build/config/linux/gtk2:gtkprint2",
+    ]
+  }
 }