Make gtk2/gtk3 compile time switchable.

build/config/linux/gtk/BUILD.gn

-# Copyright 2015 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.
-
-import("//build/config/linux/pkg_config.gni")
-
-assert(is_linux, "This file should only be referenced on Linux")
-
-# Depend on //build/config/linux/gtk to use GTK.
-#
-# GN doesn't check visibility for configs so we give this an obviously internal
-# name to discourage random targets from accidentally depending on this and
-# bypassing the GTK target's visibility.
-pkg_config("gtk_internal_config") {
-  # Gtk requires gmodule, but it does not list it as a dependency in some
-  # misconfigured systems.
-  packages = [
-    "gmodule-2.0",
-    "gtk+-2.0",
-    "gthread-2.0",
-  ]
-}
-
-# Basically no parts of Chrome should depend on GTK. To prevent accidents, the
-# parts that explicitly need GTK are whitelisted on this target.
-group("gtk") {
-  visibility = [
-    "//chrome/browser/ui/libgtk2ui",
-    "//gpu/gles2_conform_support:gles2_conform_test_windowless",
-    "//remoting/host",
-    "//remoting/host/it2me:remote_assistance_host",
-    "//remoting/host:remoting_me2me_host_static",
-  ]
-  public_configs = [ ":gtk_internal_config" ]
-}
-
-# Depend on "gtkprint" to get this.
-pkg_config("gtkprint_internal_config") {
-  packages = [ "gtk+-unix-print-2.0" ]
-}
-
-group("gtkprint") {
-  visibility = [ "//chrome/browser/ui/libgtk2ui" ]
-  public_configs = [ ":gtkprint_internal_config" ]
-}